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

[Fangrui Song via cfe-commits]

MaskRay wrote:

There may be an example that the current -mrelax-all default penalizes Thumb/Thumb2 code size as well, but I haven't found an example.

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