[flang] build broken on FreeBSD

[brooksdavis]

Since ff862d6, the flang build has been broken on FreeBSD because __FE_DENORM isn't defined on x86_64 for at least the clang 18 in the FreeBSD 14.2 base system.

See also #121949 (comment)

I've worked around it in FreeBSD's llvm-devel port with an #if !defined(__FreeBSD__), but it would nice to ship llvm 20 without a patch.

[vdonaldson]

#125087

[brad0]

This was fixed by #125213

[DavidTruby]

This is broken in a similar way on macOS/aarch64; I'm not sure what the right fix is there (there's no fpu_control.h). Should I open a separate issue for this or does anyone here know the fix for it?