base: Replace a "panic" in cprintf with an M5_UNREACHABLE.

base: Replace a "panic" in cprintf with an M5_UNREACHABLE.

The panic was just to signal that a point in the code should be
unreachable, and brought with it a thread of dependencies which would
bring in more and more extra files as it was followed.

Change-Id: I46fb99b91929dca78a6547bdc7635aab9a63a9f3
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40615
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>