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

[Nick Desaulniers via llvm-commits]

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

Will merge once presubmit returns back green! Thanks for the patch!

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