[X86] Don't lower f16->f80 fpext to libcall on darwin.

We don't provide __extendhfxf2, and only have the soft-float
__extendhfsf2 in compiler-rt.  This only changed recently with
655ba9c8, so this patch reverts back to the previous behavior.

However, the f80->f16 fptrunc is not easily implementable without
the compiler-rt __truncxfhf2, but that has always been true, and
isn't an immediate regression.

Patch by Ahmed Bougacha.

rdar://102194995