Avoid `-Wunused-function` on macOS for some `Float16` intrinsics (#43174)
We're defining `half_to_float` and `float_to_half` unconditionally but
only ever calling them on non-Mac platforms, so Clang produces unused
function warnings during the build on macOS. To fix this, we can just
move the platform check to encompass the definitions in addition to the
uses.