Add fenv.h header

Summary:
Some implementations of fenv.h use macros to define the functions they provide. This can cause problems when `std::fegetround()` is spelled in source.

This patch adds a `fenv.h` header to libc++ for the sole purpose of turning those macros into real functions.

Reviewers: rsmith, mclow.lists, ldionne

Reviewed By: rsmith

Subscribers: mgorny, christof, libcxx-commits

Differential Revision: https://reviews.llvm.org/D57729

llvm-svn: 353767