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

Provide C23 `fetestexceptflag` function according to 7.6.4.6 in the
latest [revision of the C
standard](https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3096.pdf)
from 2023-04-02.

Closes https://github.com/llvm/llvm-project/issues/87565.