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

[via llvm-commits]

https://github.com/lntue requested changes to this pull request.

Can you add some simple test cases to `llvm-project/compiler-rt/test/builtins/Unit/extendhfxf2_test.c`?

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