[compiler-rt] Enable building 16 bit floating point builtins in apple platforms if supported by the compiler (PR #84192)

[Tim Northover via llvm-commits]

https://github.com/TNorthover approved this pull request.

I think this looks sensible. It's generally pretty harmless to have these functions available even if their uses are a bit obscure and I can't think of a way this might break.

https://github.com/llvm/llvm-project/pull/84192