Add multithreading test for SamplingCircularQueue, fix implementation.

Add multithreading test for SamplingCircularQueue, fix implementation.

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.

Committed: http://code.google.com/p/v8/source/detail?r=4207

[Søren Thygesen Gjesse, 2010-03-22 14:12:29]

LGTM