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.
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