Implement CPUFeatures::InferFromOS for Linux.

This also includes CPUFeatures::InferFromIDRegisterEmulation since that is the
mechanism used for recent features in Linux.

This has been checked against the documented behaviours, but has not been tested
for many of the recent features.

Change-Id: I94d9ee05f225f12f5eb72f9d30f23ec236a38d53