8214271: Fast primitive to wake many threads

[Robbin Ehn]

Hi all, please review.

When a safepoint is ended we need a way to get back to 100% utilization as fast
as possible. 100% utilization means no idle cpu in the system if there is a
JavaThread that could be executed. The traditional ways to wake many, e.g.
semaphore, pthread_cond, is not implemented with a single syscall instead they
typical do one syscall per thread to wake.

This change-set contains that primitive, the WaitBarrier, and a gtest for it.
No actual users, which is in coming patches.

The WaitBarrier solves by doing a cooperative semaphore posting, threads woken
will also post. On Linux we can instead directly use a futex and with one
syscall wake all. Depending on how many threads and cpus the performance vary,
but a good utilization of the machine, just on the edge of saturated, the time 
to reach 100% utilization is around 3 times faster with the WaitBarrier (where 
futex is faster than semaphore).

Webrev:
http://cr.openjdk.java.net/~rehn/8214271/webrev/

CR:
https://bugs.openjdk.java.net/browse/JDK-8214271

Passes 100 iterations of gtest on our platforms, both fastdebug and release.
And have been stable when used in safepoints (t1-8) (coming patches).

Thanks, Robbin