It is possible for spurious interrupt(s) to be posted to an AP cpu

It is possible for spurious interrupt(s) to be posted to an AP cpu
while the cpu is being bootstrapped.  Explicitly enable interrupts
while still in a critical section near the end of ap_init to allow
the cpu to issue the interrupt vector(s), then clear the software
interrupt pending fields in the globaldata structure.

This stops the single report to the console of a spurious interrupt
occuring during AP booting.  This problem is unrelated to the multitude
of spurious interrupts that can be reported during device initialization.
Those will probably still occur.