arch-arm: Fix implicit fallthrough build errors

arch-arm: Fix implicit fallthrough build errors

1942b21713 introduced implicit-fallthrough errors when compiled with
GCC 8.  This change adds M5_UNREACHABLE in the default case.

Change-Id: I220f2b3fe39b5c3a65c0dd390915bffeafb28962
Signed-off-by: Chun-Chen TK Hsu <chunchenhsu@google.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20268
Reviewed-by: Jordi Vaquero <jordi.vaquero@metempsy.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>