Disable FTZ/DAZ when compiling shared libraries by default. (#80475)

This fixes https://github.com/llvm/llvm-project/issues/57589, and aligns
Clang with the behavior of current versions of gcc. There is a new
option, -mdaz-ftz, to control the linking of the file that sets FTZ/DAZ
on startup, and this flag is on by default if -ffast-math is present and
-shared isn't.

This also partially reverts fa7cd549 in that it disables the attempt
to set the IR denormal-fp-math attribute based on whether or not
-ffast-math is applied as it is insufficiently reliable.