[libc] implement sigsetjmp/siglongjmp for riscv (#137992)
See https://godbolt.org/z/jo7s6j7sq for compiled code.
```c++
#if __riscv_xlen == 64
#define STORE(A, B, C) "sd " #A ", %c[" #B "](" #C ")\n\t"
#define LOAD(A, B, C) "ld " #A ", %c[" #B "](" #C ")\n\t"
#elif __riscv_xlen == 32
#define STORE(A, B, C) "sw " #A ", %c[" #B "](" #C ")\n\t"
#define LOAD(A, B, C) "lw " #A ", %c[" #B "](" #C ")\n\t"
#else
#error "Unsupported RISC-V architecture"
#endif
namespace LIBC_NAMESPACE_DECL {
[[gnu::naked]]
LLVM_LIBC_FUNCTION(int, sigsetjmp, (sigjmp_buf, int)) {
// clang-format off
asm("beqz a1, .Lnosave\n\t"
STORE(ra, retaddr, a0)
STORE(s0, extra, a0)
"mv s0, a0\n\t"
"call %c[setjmp]\n\t"
"mv a1, a0\n\t"
"mv a0, s0\n\t"
LOAD(s0, extra, a0)
LOAD(ra, retaddr, a0)
"tail %c[epilogue]\n"
".Lnosave:\n\t"
"tail %c[setjmp]"
// clang-format on
::[retaddr] "i"(offsetof(__jmp_buf, sig_retaddr)),
[extra] "i"(offsetof(__jmp_buf, sig_extra)), [setjmp] "i"(setjmp),
[epilogue] "i"(sigsetjmp_epilogue)
: "a0", "a1", "s0");
}
```
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