pthread_barrier_test: Reduce sleep times and increase cycles.

This test now reliably reproduces the deadlock reported in #15736.

libroot: Implemented pthread barriers

This is an implementation of pthread barriers pursuant to the relevant specification.

Barriers are essentially a special case of conditional variables,
such that all threads waiting on one are woken up when the number of
waiters reaches a number provided at the initialization of the barrier.
In view of that, this implementation mimics the implementation of pthread_cond,
except it is more specialized and self-contained.

Signed-off-by: Jérôme Duval <jerome.duval@gmail.com>