[libc][math] Refactor exp2m1f16 implementation to header-only in src/__support/math folder. (#162019)

Part of #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

GitOrigin-RevId: 0d77cba6e165283283431f0d9566d0e0e06e63d7
8 files changed