[compiler-rt] Add extendhfxf2 into compiler rt (PR #113897)

[Alexander Richardson via llvm-commits]

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@@ -0,0 +1,71 @@
+// RUN: %clang_builtins %s %librt -o %t && %run %t
+// REQUIRES: librt_has_extendhfxf2
+
+#include <limits.h>
+#include <math.h> // for isnan, isinf
+#include <stdio.h>
+
+#if __LDBL_MANT_DIG__ >= 64 && defined(COMPILER_RT_HAS_FLOAT16)
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arichardson wrote:

```suggestion
#include "int_lib.h"

#if HAS_80_BIT_LONG_DOUBLE
```

This is safer 

https://github.com/llvm/llvm-project/pull/113897