Avoid -Wunused-function on macOS for some Float16 intrinsics (Jul…

…iaLang#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.

src/runtime_intrinsics.c

@@ -15,7 +15,9 @@
 const unsigned int host_char_bit = 8;
 
 // float16 intrinsics
-// TODO: use LLVM's compiler-rt
+// TODO: use LLVM's compiler-rt on all platforms (Xcode already links compiler-rt)
+
+#if !defined(_OS_DARWIN_)
 
 static inline float half_to_float(uint16_t ival) JL_NOTSAFEPOINT
 {
@@ -186,8 +188,6 @@ static inline uint16_t float_to_half(float param) JL_NOTSAFEPOINT
  return h;
 }
 
-#if !defined(_OS_DARWIN_) // xcode already links compiler-rt
-
 JL_DLLEXPORT float __gnu_h2f_ieee(uint16_t param)
 {
  return half_to_float(param);