misc: Replace M5_UNREACHABLE with GEM5_UNREACHABLE.

Change-Id: Id4cbdb8ae58c1077411e01579ac3a2d72b3b7465
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/45238
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
11 files changed