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

[Andy Kaylor via cfe-commits]

andykaylor wrote:

> > (Sidenote: "dynamic" isn't even [documented](https://clang.llvm.org/docs/UsersManual.html#cmdoption-fdenormal-fp-math)).
> 
> It's not a selectable enum of the Clang `-fdenormal-fp-math` flag, but it is one for the LLVM function attribute `denormal-fp-math`.

This is also a bit of a problem. After inlining we can have instructions with fast-math disabled appearing in the middle of functions that were compiled with fast-math enabled. What happens to the "denormal-fp-math" attribute in such cases? I wanted to show this in compiler explorer, but "#pragma float_control()" doesn't update the "denormal-fp-math attribute.

https://godbolt.org/z/KEP3YEEe9

I may have mentioned a few times that I don't like function attributes controlling fast-math behaviors.

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