[libc] [llvm] [libc] Add proxy headers for fenv types. (PR #88467)

[Nick Desaulniers via llvm-commits]

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

looks great! nice job, thank you!

need us to merge?  if so will do so next week (prefer not to merge Friday afternoon when it's time for 🍻 

https://github.com/llvm/llvm-project/pull/88467