arch, base, cpu, gpu, mem: Replace assert(0 or false with panic.

Neither assert(0) nor assert(false) give any hint as to why control
getting to them is bad, and their more descriptive versions,
assert(0 && "description") and assert(false && "description"), jury
rig assert to add an error message when the utility function panic()
already does that directly with better formatting options.

This change replaces that flavor of call to assert with panic, except
in the actual code which processes the formatting that panic uses (to
avoid infinitely recurring error handling), and in some *.sm files
since I don't know what rules those have to follow and don't want to
accidentaly break them.

Change-Id: I8addfbfaf77eaed94ec8191f2ae4efb477cefdd0
Reviewed-on: https://gem5-review.googlesource.com/c/14636
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>

ruby: Add M5_VAR_USED before variables used only inside assert in garnet2.0.
This removes errors when building gem5.fast

ruby: garnet2.0
Revamped version of garnet with more optimized single-cycle routers,
more configurability, and cleaner code.