arch-arm: Fixing implicit fallthrough build errors

arch-arm: Fixing implicit fallthrough build errors

2c242d6 introduced implicit-fallthrough errors when building against
ARM.

Added "default: return new Unknown(machInst);" to offending switch
statements; please verify this is the corret behavior

Signed-off-by: Ryan Gambord
Change-Id: I5f5e3661ec562d4a3b2699e07d1195e6877ff959
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17071
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>