The Mutex implementation is the same for all 6 POSIX platformats, just
like of them use the sched_yield() to implement Thread::YieldCPU().

R=dslomov@chromium.org

Review URL: https://codereview.chromium.org/18335008

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15623 ce2b1a6d-e550-0410-aec6-3dcde31c8c00