Fix --disable-spinlocks in 9.2 and 9.3 branches.

My back-patch of the 9.4-era commit 44cd47c1 into 9.2 and 9.3 fixed
HPPA builds as expected, but it broke --disable-spinlocks builds, because
the dummy spinlock is initialized before the underlying semaphore
infrastructure is alive.  In 9.4 and up this works because of commit
daa7527a, which decoupled initialization of an slock_t variable
from access to the actual system semaphore object.  The best solution
seems to be to back-port that patch, which should be a net win anyway
because it improves the usability of --disable-spinlocks builds in the
older branches; and it's been out long enough now to not be worrisome
from a stability perspective.