common: avoid using pthread native handle if not available

common: avoid using pthread native handle if not available

Especially when targeting Windows, llvm may not necessarily
use pthreads for std::thread. In this case, we must not use the
"native" thread handle with the pthreads API.

We'll update the ceph_pthread_getname and ceph_pthread_setname
wrappers, adding a new one: ceph_pthread_kill.

Signed-off-by: Lucian Petrut <lpetrut@cloudbasesolutions.com>