sim: Convert GuestABI example signatures to comments.

sim: Convert GuestABI example signatures to comments.

In the base Result and Argument templates, there were private static
functions which weren't meant to be used, but which would act as
documentation for what those functions should look like. They were
marked as private to prevent them from being accidentally used and
causing confusing, hard to debug errors.

Unfortunately, that also meant that those functions exist, and
apparently cause inconsistent problems with SFINAE. I assume if the
functions don't exist at all, then SFINAE will work properly. When
they're private, that seems to cause a substitution failure which
actually is an error which makes the build fail.

Change-Id: I326e9e1d05eafe1b00732ae10264354b07426e74
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28308
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>