arch,base,cpu,dev: Get rid of the M5_DUMMY_RETURN macro.

arch,base,cpu,dev: Get rid of the M5_DUMMY_RETURN macro.

This macro probably would have been defined to "return" in some cases,
to be put after a call to a function that doesn't return so that the
compiler wouldn't think control would reach the end of a non-void
function. It was only ever defined to expand to nothing, and now that
[[noreturn]] is a standard attribute, it should never be needed going
forward.

Change-Id: I37625eab72deeaede77f9347116b9fddd75febf7
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35217
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>