spinlock: add generic spinlock implementation

Adds a ceph_spinlock_t implementation that will use pthread_spinlock_t
if available, and otherwise reverts to pthread_mutex_t. Note that this
spinlock is not intended to be used in process-shared memory.

Switches implementation in:

  ceph_context
  SimpleMessenger
  atomic_t

Only ceph_context initialized its spinlock with PTHREAD_PROCESS_SHARED.
However, there does not appear to be any instance in which CephContext
is allocated in shared memory, and thus can use the default private
memory space behavior.

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>