ctfmerge: fix segfault when building on macOS

The barrier code was using semaphores which have been deprecated in
macOS and not working at all, causing a race condition. Since macOS
does not have pthread_barrier_*(), this change uses a condition
variable instead.

PR: 290958
Reported by: wosch
MFC after: 2 weeks
Reviewed by: imp, markj
Differential Revision: https://reviews.freebsd.org/D54018