[libc] [llvm] [libc][math][c23] Add fmaf16 C23 math function. (PR #130757)

[via llvm-commits]

https://github.com/overmighty commented:

We can remove some includes from libc/test/src/math/smoke/FmaTest.h now that the rounding issues should be solved by `fputil::cast`. Other than that, LGTM.

https://github.com/llvm/llvm-project/pull/130757