util: Add a helpful macro for merging two tokens to m5ops.h.

util: Add a helpful macro for merging two tokens to m5ops.h.

I've needed this type of macro often when writing code which uses the
M5OP_FOREACH macro, and so rather than re-write it each time, lets just
put a version here. This is such a common type of macro to need that
you'd think it would be part of a standard header somewhere, but to my
knowledge it isn't.

Change-Id: I0df0d9d2fd7ce1b9e6c355d6e4db197ccfff7c35
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27239
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>