machine: fix a case of undefined behaviour with SP handling

[compnerd]

The use of asm for register aliasing is supported in two different contexts:

• local variables (including GNU expression statements) where it may only be used for specifying registers for input and output operands to extended asm syntax.

  c.f. https://gcc.gnu.org/onlinedocs/gcc/Local-Register-Variables.html#Local-Register-Variables

• global variables where it may be used to observe the contents of a register.

  c.f. https://gcc.gnu.org/onlinedocs/gcc/Global-Register-Variables.html#Global-Register-Variables

The two options here is to either to hoist the variable out into a global variable, but then it should not be in a header due to fears of ODR in case the optimizer does not inline it away, and thus becomes a bit more tricky. The alternative that this change actually adopts is to explicitly use a move to copy the value out via the GNU extended assembly syntax.

With this change, it is now possible to build the Proxy Kernel completely with clang/LLVM and link with LLD. The generated kernel also runs under SPIKE and behaves as expected in a simple smoke test (without any executable prints the expected message, and runs a trivial RVV example).

[aswaterman]

Cool.

[jrtc27]

This should have used __builtin_frame_address(0) as we did downstream and failed to upstream (https://github.com/CTSRD-CHERI/riscv-pk/commit/595d003bb1ea4134caefb930e48a6e5135477489). It avoids the use of opaque inline assembly and is more portable (since for CHERI the stack pointer is a capability and thus in csp not sp).

[aswaterman]

Ah, nice.