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.

This backports !6693 (merged) to to CMake 3.21 and 3.22 branches.

Backport: release