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

[Nick Desaulniers via llvm-commits]

https://github.com/nickdesaulniers commented:

Do you mind regenerating the docs for fenv.h?

`./libc/utils/docgen/docgen.py fenv.h > libc/docs/fenv.rst`

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