[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

NOKEYCHECK=True
GitOrigin-RevId: 947df33d3d68e61a45a74c98cc3fc9b5ee2ad8de
8 files changed