[libc] [llvm] [libc][c23][fenv] Implement fetestexceptflag (PR #87828)

[Robin Caloudis via llvm-commits]

robincaloudis wrote:

Thanks for the review, @nickdesaulniers and @lntue. I fully agree on the things you pointed out and made the corresponding changes. As https://github.com/llvm/llvm-project/pull/88467 is already approved, let's wait until it is merged and use `#include "hdr/types/fexcept_t.h"` instead of `#include <fenv.h>`. 

https://github.com/llvm/llvm-project/pull/87828