Add missing functions to our stdio.c

[krtab]

As highlighted in #269 in sv-benchmarks/c/busybox-1.22.0/od-4.i we fail with

wasm-ld-18: error: /tmp/od-4-02ca46.o: undefined symbol: stdin
wasm-ld-18: error: /tmp/od-4-02ca46.o: undefined symbol: setbuf
wasm-ld-18: error: /tmp/od-4-02ca46.o: undefined symbol: fread
wasm-ld-18: error: /tmp/od-4-02ca46.o: undefined symbol: stdin
wasm-ld-18: error: /tmp/od-4-02ca46.o: undefined symbol: fseek
wasm-ld-18: error: /tmp/od-4-02ca46.o: undefined symbol: ferror
wasm-ld-18: error: /tmp/od-4-02ca46.o: undefined symbol: stdin
wasm-ld-18: error: /tmp/od-4-02ca46.o: undefined symbol: ferror
wasm-ld-18: error: /tmp/od-4-02ca46.o: undefined symbol: fgetc
wasm-ld-18: error: /tmp/od-4-02ca46.o: undefined symbol: fread
wasm-ld-18: error: /tmp/od-4-02ca46.o: undefined symbol: ferror
wasm-ld-18: error: /tmp/od-4-02ca46.o: undefined symbol: stdin
clang: error: linker command failed with exit code 1 (use -v to see invocation)
run ['/usr/bin/clang' '-O0' '--target=wasm32' '-m32' '-ffreestanding'
     '--no-standard-libraries' '-Wno-everything' '-flto=thin'
     '-Wl,--entry=main' '-Wl,--export=main' '-Wl,--lto-O0'
     '-Wl,-z,stack-size=8388608' '-I'
     '/home/zapashcanon/.config/opam/5.1.0/share/owi/libc' '-o' 'a.out.wasm'
     '/home/zapashcanon/.config/opam/5.1.0/share/owi/binc/libc.wasm'
     'owi-out/od-4.c']: exited with 1

This is because we are missing functions in our implementation of stdio.h

[zapashcanon]

Actually it looks like we already have these functions. The file is actually not including stdio.h, but has stuff like:

extern struct _IO_FILE *stdin;

I'm not sure how we should deal with it. Maybe adding an option to clang not to fail on undefined symbols (I remember seeing one at some point) would be enough...