Enable interrupts on the BSP once all PICs are initialized.

This moves the enabling of interrupts slightly earlier (the old location
was still before devices were enumerated and probed) and does it in the
interrupt code (rather than in the device configuration code). This
also avoids tripping over an assertion on the first TLB shootdown with
earlier AP startup.

Reviewed by: kib
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D5710