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

[via llvm-commits]

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@@ -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) {
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lntue wrote:

I think this should be in a separate `extendhfxf2.c` file.  This file is for converting `half -> single`.

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