util: Add support for multiple call types in the m5 utility.

util: Add support for multiple call types in the m5 utility.

Using mechanisms added in previous CLs, this change modifies the m5
utility so that it can use any of the back ends enabled and implemented
by each variant, defaulting to one particular implementation if not is
selected explicitly.

On x86, the default mechanism is the magic address. All other variants
default to the magic instruction since they don't have a well
established address to use or even in most cases an implementation to
use.

The ability to override the particular magic address the utility wants
to use (necessary on variants such as aarch64) will be added in a future
CL.

Change-Id: I5fc414740e30759e7dde719cddcc8d5d41f8cc74
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27242
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Pouya Fotouhi <pfotouhi@ucdavis.edu>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>