util: Add an abstraction layer for call types in the m5 utility.

util: Add an abstraction layer for call types in the m5 utility.

These make the calling code in m5.c a little bit more generic. Each call
type will have a function to check the arguments and see if that type is
being requested and/or has any additional options set in the arguments.
If so, those are processed, and argc and argv are adjusted.

Then another function returns the appropriate dispatch table to use for
that invocation scheme. This is behind a function instead of, for
instance, a global variable because it gives the call type a little bit
more control over what's happening, for instance if it would use
different implementations in slightly different circumstances.

Change-Id: I661cf202ec657466496767cbdf331fe27995ab26
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27241
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>