arm64: Don't enable interrupts in init_secondary().

arm64: Don't enable interrupts in init_secondary().

Doing so can cause deadlocks or panics during boot, if an interrupt
handler accesses uninitialized per-CPU scheduler structures.  This seems
to occur frequently when running under QEMU or AWS.  The idle threads
are set up to release a spinlock section and enable interrupts in
fork_exit(), so there is no need to enable interrupts earlier.

Reviewed by:	kib
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D23328