FindThreads: Revert libc symbol check to pthread_create
Since !2629 (merged) we check libc for
pthread_kill instead of
pthread_create. However, on FreeBSD
pthread_kill is in libc but not
pthread_create. Discussion in the original merge request for the above commit also considered
pthread_attr_init. Every symbol seems to have some reason it is not an appropriate choice.
Revert to the pre-3.14 behavior of using
pthread_create pending further investigation.
Fixes: #18984 (closed)