[libc][math] Refactor tanhf16 implementation to header-only in src/__support/math folder. (#178645) Part of https://github.com/llvm/llvm-project/issues/147386 closed https://github.com/llvm/llvm-project/issues/178492 GitOrigin-RevId: a4cb2b00df606a6fd42dd4a8273e61fc8696939a