[libc] Raise x87 exceptions by synchronizing with "fwait".

Couple of helper functions enableExcept and disableExcept have been
added. In a later round, they will be used to implemented the GNU
extension functions feenableexcept and fedisableexcept.

Differential Revision: https://reviews.llvm.org/D92821

GitOrigin-RevId: ab3cbe4bc0d952ca58a3e2263629591ef898de3f
5 files changed