arch-arm: Report real instruction encoding when Undefined

arch-arm: Report real instruction encoding when Undefined

When dumping the opcode that caused an Undefined Instruction, we just
want to dump the real instruction encoding, and not the extended version
with metabits (like thumb, bigThumb etc). This was not appening when
panicking in SE mode.

The patch is also replacing custom masking in the Unknown(64) disassembler
in favour of ArmStaticInstruction::encoding() helper.

Change-Id: I9eb6fd145d02b4b07bb51f0bd89ca014d6d5a6de
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18395
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>