scripts/decodecode: set ARCH when running natively on arm/arm64

scripts/decodecode: set ARCH when running natively on arm/arm64

When running decodecode natively on arm64, ARCH is likely not to be set,
and we end-up with .4byte instead of .inst when generating the
disassembly.

Similar effects would occur if running natively on a 32bit ARM platform,
although that's even less popular.

A simple workaround is to populate ARCH when it is not set and that we're
running on an arm/arm64 system.

Link: http://lkml.kernel.org/r/20181210174659.31054-2-marc.zyngier@arm.com
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>