Cleaned up CpuFeature scope handling.

First of all, it has nothing to do with Isolates, it is related to the assembler
at hand. Furthermore, the saving/restoring is platform-independent. Cleaned up
some platform-specific stuff on the way.

Note that there are some things which still need some cleanup, like e.g. using
EnumSet instead of uint64_t, making Probe() more uniform across platforms etc.,
but the CL is already big enough.

BUG=v8:2487

Review URL: https://codereview.chromium.org/12391055

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13823 ce2b1a6d-e550-0410-aec6-3dcde31c8c00