Call ast when handling irq from userspace, otherwise we could miss
reschedule. Right now arm_cpu_intr() does critical_exit() as the last
action, so the impact is not serious.

Remove duplicated interrupt disable in restore_registers macro, when
returning to usermode. The do_ast macro disabled interrupts for us.

Reviewed by: andrew
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D3714