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

[Joshua Cranmer via cfe-commits]

jcranmer-intel wrote:

I did a thorough investigation into how the `denormal-fp-math=preserve-sign,preserve-sign` attribute affects the resulting IR for all of the SPEC benchmarks (which actually do run into subnormals), and the basic summary I found is that the only differences I found were that the FTZ/DAZ mode prevented a few inferences that are still true in denormal-flushing: things like `x >= 0` implies `x >= -1`.

So overall, the practical effect of the `denormal-fp-math` attribute being set incorrectly doesn't appear to matter.

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