Fix issue 410: test-log/ProfLazyMode flakinness under Linux.

Fix issue 410: test-log/ProfLazyMode flakinness under Linux.

I found two causes of flakinness:
 - SIGPROF signal isn't delivered to a process;
 - Profiler thread (the one that retrieves tick events from
   the queue and writes to log) doesn't get a CPU;
Both are fixed.

The script from bug description with run count increased to 200 runs without any test failures.

OS X and Windows are unaffected because they don't use signals mechanism.

BUG=http://code.google.com/p/v8/issues/detail?id=410
TEST=see bug description

Review URL: http://codereview.chromium.org/159406

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2547 ce2b1a6d-e550-0410-aec6-3dcde31c8c00