testsuite/pthread4.cc and linux

[Benjamin Kosnik]

>As committed to mainline (Benjamin, please move to 3.3 after you
>verified that it helped in context and reopened).  I ran with various
>gcc versions on various platforms and noted 2x-3x speedup as expected
>(even for an optimally-fair, time-based scheduler, I'd expect this
>code to have done at least twice the extra work required)...  If your
>platform still fails this test and/or takes a really long time after
>this patch, then we have a real issue to debug (i.e. a principle of
>the platform/thread implementation should care about this issue).

Ok, all good now. Thanks!

-benjamin