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

[Andy Kaylor via cfe-commits]

andykaylor wrote:

> One final nit before you submit, though: please update the PR description and the release notes to also mention that x86 no longer modifies `-fdenormal-fp-math` based on `-ffast-math`.

The Clang User's Manual makes some contradictory statements about this. In one place it says, `-fno-fast-math` resets `-fdenormal-fp-math=<value>` to its target-dependent default, but just a few lines later it says "-fno-fast-math implies -fdenormal-fp-math=ieee" which isn't true for all targets. It says `-fno-unsafe-math-optimizations` implies '-fdenormal-fp-math=ieee` which, unfortunately, is currently be true.

The User's Manual also says that `-ffp-model=fast` sets `denormal_fp_math` to IEEE, which is currently true, but really shouldn't be.

I'm working on a PR to fix the fp-model handling, which I'll update after this PR lands. I can fix the other issues I've mentioned here in that patch. There are a few points that I am not quite clear on. I'm going to start a Discourse topic to summarize what we've agreed on here and discuss the things I'm not sure about.

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