Toolchains can't be moved, as some paths appear to be hardcoded

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When testing a build for aarch64 I noticed that the paths seem to be hardcoded, so toolchains can't be moved to a directory other than where they were built at. As you can see, aarch64-linux-musl-gcc works when invoked at mussel's default path:

andy:~/code/mussel/toolchain/bin$ ./aarch64-linux-musl-gcc test.c -o test -s -static -v
Using built-in specs.
COLLECT_GCC=./aarch64-linux-musl-gcc
COLLECT_LTO_WRAPPER=/home/andy/code/mussel/toolchain/libexec/gcc/aarch64-linux-musl/13.2.0/lto-wrapper
Target: aarch64-linux-musl
Configured with: /home/andy/code/mussel/sources/gcc/gcc-13.2.0/configure --prefix=/home/andy/code/mussel/toolchain --target=aarch64-linux-musl --with-sysroot=/home/andy/code/mussel/sysroot --enable-languages=c,c++ --disable-multilib --disable-bootstrap --disable-libsanitizer --disable-werror --enable-initfini-array --with-arch=armv8-a --with-abi=lp64 --enable-fix-cortex-a53-835769 --enable-fix-cortex-a53-843419
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 13.2.0 (GCC) 
COLLECT_GCC_OPTIONS='-o' 'test' '-s' '-static' '-v' '-march=armv8-a' '-mlittle-endian' '-mabi=lp64'
 /home/andy/code/mussel/toolchain/libexec/gcc/aarch64-linux-musl/13.2.0/cc1 -quiet -v test.c -quiet -dumpbase test.c -dumpbase-ext .c -march=armv8-a -mlittle-endian -mabi=lp64 -version -o /tmp/cc9VhAz8.s
GNU C17 (GCC) version 13.2.0 (aarch64-linux-musl)
    compiled by GNU C version 13.2.0, GMP version 6.3.0, MPFR version 4.2.1, MPC version 1.3.1, isl version isl-0.26-GMP

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
ignoring nonexistent directory "/home/andy/code/mussel/sysroot/usr/local/include"
#include "..." search starts here:
#include <...> search starts here:
 /home/andy/code/mussel/toolchain/lib/gcc/aarch64-linux-musl/13.2.0/../../../../aarch64-linux-musl/include
 /home/andy/code/mussel/sysroot/usr/include
 /home/andy/code/mussel/toolchain/lib/gcc/aarch64-linux-musl/13.2.0/include
End of search list.
Compiler executable checksum: c3f9714356a1cc7be1615938ff7e0062
COLLECT_GCC_OPTIONS='-o' 'test' '-s' '-static' '-v' '-march=armv8-a' '-mlittle-endian' '-mabi=lp64'
 /home/andy/code/mussel/toolchain/lib/gcc/aarch64-linux-musl/13.2.0/../../../../aarch64-linux-musl/bin/as -v -EL -march=armv8-a -mabi=lp64 -o /tmp/ccft0xy7.o /tmp/cc9VhAz8.s
GNU assembler version 2.42 (aarch64-linux-musl) using BFD version (GNU Binutils) 2.42
COMPILER_PATH=/home/andy/code/mussel/toolchain/libexec/gcc/aarch64-linux-musl/13.2.0/:/home/andy/code/mussel/toolchain/libexec/gcc/aarch64-linux-musl/13.2.0/:/home/andy/code/mussel/toolchain/libexec/gcc/aarch64-linux-musl/:/home/andy/code/mussel/toolchain/lib/gcc/aarch64-linux-musl/13.2.0/:/home/andy/code/mussel/toolchain/lib/gcc/aarch64-linux-musl/:/home/andy/code/mussel/toolchain/lib/gcc/aarch64-linux-musl/13.2.0/../../../../aarch64-linux-musl/bin/
LIBRARY_PATH=/home/andy/code/mussel/toolchain/lib/gcc/aarch64-linux-musl/13.2.0/:/home/andy/code/mussel/toolchain/lib/gcc/aarch64-linux-musl/13.2.0/../../../../aarch64-linux-musl/lib/../lib/:/home/andy/code/mussel/sysroot/lib/../lib/:/home/andy/code/mussel/sysroot/usr/lib/../lib/:/home/andy/code/mussel/toolchain/lib/gcc/aarch64-linux-musl/13.2.0/../../../../aarch64-linux-musl/lib/:/home/andy/code/mussel/sysroot/lib/:/home/andy/code/mussel/sysroot/usr/lib/
COLLECT_GCC_OPTIONS='-o' 'test' '-s' '-static' '-v' '-march=armv8-a' '-mlittle-endian' '-mabi=lp64' '-dumpdir' 'test.'
 /home/andy/code/mussel/toolchain/libexec/gcc/aarch64-linux-musl/13.2.0/collect2 -plugin /home/andy/code/mussel/toolchain/libexec/gcc/aarch64-linux-musl/13.2.0/liblto_plugin.so -plugin-opt=/home/andy/code/mussel/toolchain/libexec/gcc/aarch64-linux-musl/13.2.0/lto-wrapper -plugin-opt=-fresolution=/tmp/ccrj2Upo.res -plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lgcc_eh -plugin-opt=-pass-through=-lc --sysroot=/home/andy/code/mussel/sysroot -Bstatic -X -EL -maarch64linux --fix-cortex-a53-835769 --fix-cortex-a53-843419 -o test -s /home/andy/code/mussel/sysroot/usr/lib/../lib/crt1.o /home/andy/code/mussel/sysroot/usr/lib/../lib/crti.o /home/andy/code/mussel/toolchain/lib/gcc/aarch64-linux-musl/13.2.0/crtbeginT.o -L/home/andy/code/mussel/toolchain/lib/gcc/aarch64-linux-musl/13.2.0 -L/home/andy/code/mussel/toolchain/lib/gcc/aarch64-linux-musl/13.2.0/../../../../aarch64-linux-musl/lib/../lib -L/home/andy/code/mussel/sysroot/lib/../lib -L/home/andy/code/mussel/sysroot/usr/lib/../lib -L/home/andy/code/mussel/toolchain/lib/gcc/aarch64-linux-musl/13.2.0/../../../../aarch64-linux-musl/lib -L/home/andy/code/mussel/sysroot/lib -L/home/andy/code/mussel/sysroot/usr/lib /tmp/ccft0xy7.o --start-group -lgcc -lgcc_eh -lc --end-group /home/andy/code/mussel/toolchain/lib/gcc/aarch64-linux-musl/13.2.0/crtend.o /home/andy/code/mussel/sysroot/usr/lib/../lib/crtn.o
COLLECT_GCC_OPTIONS='-o' 'test' '-s' '-static' '-v' '-march=armv8-a' '-mlittle-endian' '-mabi=lp64' '-dumpdir' 'test.'
andy:~/code/mussel/toolchain/bin$ echo $?
0

Now let's try compiling test.c after moving the toolchain into a new directory.

andy:~/code/mussel/toolchain/bin$ cd ../..
andy:~/code/mussel$ mkdir -p test_dir
mkdir: created directory 'test_dir'
andy:~/code/mussel$ mv sysroot toolchain test_dir/
renamed 'sysroot' -> 'test_dir/sysroot'
renamed 'toolchain' -> 'test_dir/toolchain'
andy:~/code/mussel$ cd test_dir/toolchain/bin
andy:~/code/mussel/test_dir/toolchain/bin$ ./aarch64-linux-musl-gcc test.c -o test -s -static -v

Since toolchain and sysroot were moved, gcc can no longer find the headers and thus fails.

Using built-in specs.
COLLECT_GCC=./aarch64-linux-musl-gcc
COLLECT_LTO_WRAPPER=/home/andy/code/mussel/test_dir/toolchain/bin/../libexec/gcc/aarch64-linux-musl/13.2.0/lto-wrapper
Target: aarch64-linux-musl
Configured with: /home/andy/code/mussel/sources/gcc/gcc-13.2.0/configure --prefix=/home/andy/code/mussel/toolchain --target=aarch64-linux-musl --with-sysroot=/home/andy/code/mussel/sysroot --enable-languages=c,c++ --disable-multilib --disable-bootstrap --disable-libsanitizer --disable-werror --enable-initfini-array --with-arch=armv8-a --with-abi=lp64 --enable-fix-cortex-a53-835769 --enable-fix-cortex-a53-843419
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 13.2.0 (GCC) 
COLLECT_GCC_OPTIONS='-o' 'test' '-s' '-static' '-v' '-march=armv8-a' '-mlittle-endian' '-mabi=lp64'
 /home/andy/code/mussel/test_dir/toolchain/bin/../libexec/gcc/aarch64-linux-musl/13.2.0/cc1 -quiet -v -iprefix /home/andy/code/mussel/test_dir/toolchain/bin/../lib/gcc/aarch64-linux-musl/13.2.0/ test.c -quiet -dumpbase test.c -dumpbase-ext .c -march=armv8-a -mlittle-endian -mabi=lp64 -version -o /tmp/ccHA1H0P.s
GNU C17 (GCC) version 13.2.0 (aarch64-linux-musl)
    compiled by GNU C version 13.2.0, GMP version 6.3.0, MPFR version 4.2.1, MPC version 1.3.1, isl version isl-0.26-GMP

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
ignoring nonexistent directory "/home/andy/code/mussel/sysroot/usr/local/include"
ignoring duplicate directory "/home/andy/code/mussel/test_dir/toolchain/bin/../lib/gcc/../../lib/gcc/aarch64-linux-musl/13.2.0/../../../../aarch64-linux-musl/include"
ignoring nonexistent directory "/home/andy/code/mussel/sysroot/usr/include"
ignoring duplicate directory "/home/andy/code/mussel/test_dir/toolchain/bin/../lib/gcc/../../lib/gcc/aarch64-linux-musl/13.2.0/include"
#include "..." search starts here:
#include <...> search starts here:
 /home/andy/code/mussel/test_dir/toolchain/bin/../lib/gcc/aarch64-linux-musl/13.2.0/../../../../aarch64-linux-musl/include
 /home/andy/code/mussel/test_dir/toolchain/bin/../lib/gcc/aarch64-linux-musl/13.2.0/include
End of search list.
Compiler executable checksum: c3f9714356a1cc7be1615938ff7e0062
test.c:1:10: fatal error: stdio.h: No such file or directory
    1 | #include <stdio.h>
      |          ^~~~~~~~~
compilation terminated.
andy:~/code/mussel/test_dir/toolchain/bin$ echo $?
1

I'll submit another PR when I get around to it as I believe I found a fix, but I figured I'd file this issue so it's accounted for.