Further reduce the number of semaphores used under --disable-spinlocks.

Further reduce the number of semaphores used under --disable-spinlocks.

Per discussion, there doesn't seem to be much value in having
NUM_SPINLOCK_SEMAPHORES set to 1024: under any scenario where you are
running more than a few backends concurrently, you really had better have a
real spinlock implementation if you want tolerable performance.  And 1024
semaphores is a sizable fraction of the system-wide SysV semaphore limit
on many platforms.  Therefore, reduce this setting's default value to 128
to make it less likely to cause out-of-semaphores problems.