Allow a platform to indicate that some CPU features are always

Allow a platform to indicate that some CPU features are always
available.  We use this to ensure that snapshots on MacOSX can
use SSE2 instructions.  Also clean up and assertify the
handling of things we can't do when we are generating a
snapshot.  Fix a bug in the new serialization tests where
they activated Snapshot::enable() too late after code had been
generated that assumed no snapshots.
Review URL: http://codereview.chromium.org/391051

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3301 ce2b1a6d-e550-0410-aec6-3dcde31c8c00