FindThreads: Honor THREADS_PREFER_PTHREAD_FLAG when pthread is found in libc

The `-pthread` flag tells the compiler/linker to link to additional
libraries needed for thread support (e.g. libatomic on riscv64). The
flag therefore should be used if requested using
`THREADS_PREFER_PTHREAD_FLAG` also when the pthread functions are
found in libc.
35 jobs for !6906 with FindThreads-libc-pthread-flag in 27 minutes and 15 seconds (queued for 3 seconds)
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