[clang] [RISCV][clang] Don't enable -mrelax-all for -O0 on RISC-V (PR #88538)

[Fangrui Song via cfe-commits]

https://github.com/MaskRay approved this pull request.

I left a comment at 
https://github.com/llvm/llvm-project/issues/87127#issuecomment-2063027106

This is more about a weird -O0 -mrelax-all behavior that RISC-V now opts out, which is good on its own, even if the basis looks like a self-inflicted problem...

https://github.com/llvm/llvm-project/pull/88538