[turbofan] Improve testability of the InstructionSelector.

Allow to pass the set of supported CPU features to
the InstructionSelector, so it can be tested without
messing with the command line flags.

Also add InstructionSelector sample for ia32.

TEST=cctest/test-instruction-selector,cctest/test-instruction-selector-{arm,ia32}
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/441883004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22876 ce2b1a6d-e550-0410-aec6-3dcde31c8c00