[libc] [llvm] [libc][math] change bf16fmal to be header-only and constexpr-compat (PR #181666)

[Muhammad Bassiouni via llvm-commits]

https://github.com/bassiounix approved this pull request.

LGTM.
Please resolve conflicts to initiate CI and merge your code.

https://github.com/llvm/llvm-project/pull/181666