[libc][math] Refactor f16sqrt to Header Only (#177167)
Fixes #175330
Part of https://github.com/llvm/llvm-project/issues/147386
in preparation for:
https://discourse.llvm.org/t/rfc-make-clang-builtin-math-functions-constexpr-with-llvm-libc-to-support-c-23-constexpr-math-functions/86450
