[libc][math] Refactor f16sqrtl to Header Only. (#176333)

builds correctly with both `clang`, `gcc`, `cmake`, and `Bazel`.

Closes https://github.com/llvm/llvm-project/issues/175331.

GitOrigin-RevId: bd3ecdc59bb69a5528dee06f1e47c733bb89a25f
8 files changed