Add barriers using MIPS 'sync' instructions as needed for SMP
systems.

BUG=246947
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/15981017

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15062 ce2b1a6d-e550-0410-aec6-3dcde31c8c00