Move cache line size calculation directly into CPU::FlushICache.

This disentagles the initialization/dependency mess quite a bit and
makes things vastly simpler. If the 'mrs' on every flush is too
expensive (which it is hopefully not), the cache line sizes will have
to be instance variables of the CPU class and FlushICache will have to
be a member function. This would involve some more or less tricky
refactorings, which we shouldn't do until we are *really* forced to
do.

BUG=359977
LOG=y
R=rodolph.perfetta@gmail.com

Review URL: https://codereview.chromium.org/269543016

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21119 ce2b1a6d-e550-0410-aec6-3dcde31c8c00