[libc] [llvm] [libc][math][c23] Add atan2f16 function (PR #183531)

[Xinlong Chen via llvm-commits]

Xinlong-Chen wrote:

@bassiounix @hulxv @lntue I've addressed the review comments. Could you please take another look when convenient?

https://github.com/llvm/llvm-project/pull/183531