[clang] Align -ffp-model=fast denormal handling with -ffast-math (PR #89477)

[Andy Kaylor via cfe-commits]

andykaylor wrote:

This patch will need extensive changes once https://github.com/llvm/llvm-project/pull/80475 lands. We're basically decoupling "denormal-fp-math" from fast-math, but I think we still want to link "crtfastmath.o" when --fp-model=fast is used, as we do with -ffast-math.

https://github.com/llvm/llvm-project/pull/89477