[clang] [Driver] Don't default to -mrelax-all for non-RISCV -O0 (PR #90013)

[Fangrui Song via cfe-commits]

MaskRay wrote:

`createAMDGPUELFStreamer` ignores `RelaxAll`, so this appears to be x86-only.

There may be some clean-up opportunities.

https://github.com/llvm/llvm-project/pull/90013