This is for the case of Linux, where sampling is done using SIGPROF
signal handler which is executed in the context of an interrupted
thread. In this case, my previous implementation with TLS doesn't
work.

Review URL: http://codereview.chromium.org/1138004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4207 ce2b1a6d-e550-0410-aec6-3dcde31c8c00