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

[Sam Clegg via cfe-commits]

sbc100 wrote:

This change seems to have caused a test in emscripten to starting failing with `argument unused during compilation: '-O2'`: https://github.com/emscripten-core/emscripten/pull/21841.

Is that an intended side effect of this change?  The fix on our end seems reasonable but posting here just in case.

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