X86: call fp16-conversion functions soft-float on Darwin.

We've been shipping implementations of these with a soft-float ABI since MacOS
10.10 in 2014 and there's evidence they're in binaries now, so we can't easily
switch to %xmm0.

This emits special libcalls with casts in place to restore the soft-float ABI
for __truncdfhf2, __truncsfhf2, and __extendhfsf2.