[libc] Extend the current fenv functions to aarch64.

This change does not try to move the common parts of x86 and aarch64 and
build few abstractions over them. While this is possible, x86 story
needs a bit of cleanup, especially around manipulation of the mxcsr
register. Moreover, on x86 one can raise exceptions without performing
exception raising operations. So, all of this can be done in follow up
patches.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D94947