arch-arm: Fix Hlt64,Svc64,Hvc64,Smc64,Brk64 disassembly

arch-arm: Fix Hlt64,Svc64,Hvc64,Smc64,Brk64 disassembly

This patch fixes the disassembly of AArch64 Exception Generating
instructions, which were not printing the encoded immediate field. This
has been accomplished by changing their underlying type to a newly
defined one.

Change-Id: If58ae3e620d2baa260e12ecdc850225adfcf1ee5
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/8368
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>