Added labelled thread names to help with some debugging activity. Right now,

Added labelled thread names to help with some debugging activity. Right now,
the only platform that it works on is linux (using the prctl API to set the
names of the threads). Other platforms are setup to build properly if the
flag is set, but their thread names are not currently set.

Patch by Mark Lam from Hewlett-Packard Development Company, LP

Review URL: http://codereview.chromium.org/6070009

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6141 ce2b1a6d-e550-0410-aec6-3dcde31c8c00