nmalloc - Fix depot race - this should fix remaining issues with firefox.

nmalloc - Fix depot race - this should fix remaining issues with firefox.

* nmalloc was accidently calling the libc _pthread_spin_lock() stub
  even when threaded, causing all depot operations to race.  No actual
  lock was being acquired.

  Change it over to the internal _SPINLOCK() API.

* Expand libthread_xu's base spinlocks from 128 to 256 and use those
  for the depot lock.  Note that the depot[] array can be upwards of
  80 elements so lets not take any chances.

Reported-by: Pierre Abbat <phma@phma.optus.nu>, others