[CMake][libc] Don't do CPU feature detection when cross-compiling

We won't be able to run the compiled program since it will be compiled
for different system. We instead allow passing the CPU features via
CMake option in that case.

Differential Revision: https://reviews.llvm.org/D95203

GitOrigin-RevId: c4819eec1a2aea4758cc1ed38aefb0f1c9dec94a
2 files changed