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

[Shengchen Kan via cfe-commits]

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

LGTM. My understanding is that this flag is used for reducing compile time and debug only.

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