FindThreads: Use complete program to test for pthreads in libc

Our check for just `pthread_create` is not sufficient because there are
cases where that symbol may be found in libc but not all of pthreads.
We first tried to address this in commit e9a1ddc594 (FindThreads:
Replace the pthread symbol checking in libc., 2018-11-18,
v3.14.0-rc1~292^2) by switching to `pthread_kill`, but that had to be
reverted by commit 18320230ec (FindThreads: Revert libc symbol check to
pthread_create, 2019-02-27, v3.14.0-rc3~6^2) because there are other
cases where `pthread_kill` is in libc but the rest of pthreads is not.
Update our check to try a complete program using pthreads as an actual
application might.

Fixes: #18994

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