[clang] Disable FTZ/DAZ when compiling shared libraries by default. (PR #80475)

[Matt Arsenault via cfe-commits]

arsenm wrote:

> I wonder if, instead, we should just have `-ffast-math` always downgrade `-fdenormal-fp-math=ieee` to `-fdenormal-fp-math=preserve-sign`, under the rationale of "you asked for fast math, and preserve-sign mode might let the compiler generate faster code"?

This could also use denormal-fp-math=dynamic. It is not always safe to run preserve-sign code under IEEE settings 

https://github.com/llvm/llvm-project/pull/80475