pegbox: spin_unlock needs a locked op (due to the relaxed MMU rules)

Otherwise, spin_lock can read 0xff, spin_unlock write 0xfe, and spin_lock
 write 0xff back - leaving the lock in the held state but no CPU holding it.
pegasus: support up to 128 CPUs

Note that using anything like that many is a really bad idea as most of them
 will spent the whole time \WAITed for RAM access.