FindThreads: Revert libc symbol check to pthread_create
Since commit e9a1ddc5 (FindThreads: Replace the pthread symbol checking in libc., 2018-11-18, v3.14.0-rc1~292^2) 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_key_create`, `pthread_self`, and `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.