Minor tweak of FakeLowp CMakefile (#36861)
Summary:
On some machine I found error like cannot find `cpuinfo.h` when building FakeLowp ops. Fixing it. Also updated the README.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36861
Reviewed By: amylittleyang
Differential Revision: D21105755
Pulled By: yinghai
fbshipit-source-id: 4f17bd969d38e1b2945b8753ffe4bdc703de36bf
