Fix a lock up where we enter swapper() with interrupts disabled.

In smp_rendezvous_cpus we expect to wait for all cpus to enter
smp_rendezvous_action. If we call this holding a proc lock swapper may
attempt to also lock it, however as interrupts are disabled the cpu never
handles the ipi. Because smp_rendezvous_action waits for all signaled
cpus before contining it may get caught waiting for the cpu running swapper
as the proc mutex will be unlocked after smp_rendezvous_cpus finishes.

The fix is to enable interrupts in the configure stage as we should be
doing.

MFC after: 1 week