1.0.4.30: make WITH-SPINLOCK-AND-WITHOUT-GCING inhibit interrupts as well

1.0.4.30: make WITH-SPINLOCK-AND-WITHOUT-GCING inhibit interrupts as well

 * Previously we could catch an interrupt while GC was inhibited, and
   hence any locks the interrupt handler was using could not know if
   they were being used with GC enabled or not => GC deadlocks.

 * Since this was detected by the runtime warning in WITH-INTERRUPTS
   about re-enabling interrupts while GC is inhibited keep it around
   in case we have more places this can happen.

 * Make MAYBE-HANDLE-PENDING-GC check for pending interrupts as well.

 * While we're at it, make WITH-SPINLOCK slightly safer: don't release
   locks we didn't obtain, and make us grab the lock inside the UWP.
   ...not 100% given in the presence of asynch unwinds, but better.

 * Also move *INTERRUPTS-ENABLED* and *INTERRUPT-PENDING* to SB-SYS.

 * Whitespace in WITHOUT-INTERRUPTS.