[libc] Add CMake script to check host cpu features

Summary:
Tested on MacOSX and Linux.
For robustness we can go the OpenCV way and add individual c++ files with intrinsics.
https://github.com/opencv/opencv/blob/master/cmake/checks/cpu_avx2.cpp

Reviewers: sivachandra, abrachet

Subscribers: mgorny, MaskRay, tschuett, libc-commits

Tags: #libc-project

Differential Revision: https://reviews.llvm.org/D74897