arch-arm: Decode Brk64 instructions

The brk instruction in aarch64 was decoded as an unimplemented
instruction. Fix that.

Change-Id: I3eb36a016ab56d882426c5cdef3a0b594de0f9cd
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jack Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/8142
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>