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 <91dcdfb9561c211c6d292d4179e3ae64e470601d@google.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20268
Reviewed-by: Jordi Vaquero <a07d54953374631b01998252a09a2652defe259a@metempsy.com>
Reviewed-by: Giacomo Travaglini <8eef13a4f970e2e9372c8d26ad0737b760f99c02@arm.com>
Reviewed-by: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>
Maintainer: Giacomo Travaglini <8eef13a4f970e2e9372c8d26ad0737b760f99c02@arm.com>
Maintainer: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5