Simplified CPU/CpuFeatures a bit.

This is a necessary intermediate step to disentangle the startup.

In the long run CPU and CpuFeatures should probably be merged,
and Serializer::enabled usage should be radically reduced, but
we're not there yet.

BUG=359977
LOG=y
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/258993002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21001 ce2b1a6d-e550-0410-aec6-3dcde31c8c00