perf bench futex: Sync waker threads

Waker threads in the futex wake-parallel benchmark are started by a loop
using pthread_create().  However, there is no synchronization for when
the waker threads wake the waiting threads.  Comparison of the waker
threads' measurement timestamps show they are not all running
concurrently because older waker threads finish their task before newer
waker threads even start.

This patch uses a barrier to better synchronize the waker threads.

Signed-off-by: James Yang <james.yang@arm.com>
Cc: Kim Phillips <kim.phillips@arm.com>
Link: http://lkml.kernel.org/r/20171127042101.3659-4-dave@stgolabs.net


Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
[ Disable the wake-parallel test for systems without pthread_barrier_t ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>