arch-arm: Fix Unknown Instruction disassemble

Do not print the entire ExtMachInst when disassembling an Unknown
Instruction.

Change-Id: Icd5908ec0fa430090165b2426372bdeb43c2a155
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/10062
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
2 files changed