[compiler-rt] Add missing extendhfxf2 in compiler rt (PR #109090)

[via llvm-commits]

================
@@ -16,6 +16,10 @@ COMPILER_RT_ABI NOINLINE float __extendhfsf2(src_t a) {
   return __extendXfYf2__(a);
 }
 
+COMPILER_RT_ABI NOINLINE long double __extendhfxf2(src_t a) {
----------------
lntue wrote:

`__extendXfYf2__` is defined in `fp_extend_impl.inc`, not in here.  You can define `SRC_HALF`, `DST_SINGLE`, and include `fp_extend_impl.inc` in that separate file.

https://github.com/llvm/llvm-project/pull/109090