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

[via llvm-commits]

Serosh-commits wrote:

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

thanks done it i would love to work more on these type of issues 

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