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