On OpenBSD 6.4 x86_64 the barrier_serial_thread test fails due to
OpenBSD's pthread_barrier_wait() implementation returning
PTHREAD_BARRIER_SERIAL_THREAD from the first thread that releases the
wait. Switch to libuv's internal implementation to address the issue.
Fixes: https://github.com/libuv/libuv/issues/2145
PR-URL: https://github.com/libuv/libuv/pull/2144
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>