Fix the CPU feature detection.

Move all of the CPU detection logic to the CPU class, and make
all other code use the CPU class for feature detection.

This also fixes the ARM CPU feature detection logic, which was
based on fragile string search in /proc/cpuinfo. Now we use
ELF hwcaps if available, falling back to sane(!!) parsing of
/proc/cpuinfo for CPU features.

The ia32 and x64 code was also cleaned up to make it usable
outside the assembler.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/23401002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16315 ce2b1a6d-e550-0410-aec6-3dcde31c8c00