sim: Implement a varargs like mechanism for GuestABI system.

This will let a function called with a GuestABI emulate the ...
mechanism available in C. To make that possible without the functions
knowing anything about the ABI and to follow C++'s (sensible)
templating and virtual function rules, you have to tell VarArgs what
types you might want to extract from it, unlike the pure ... varargs
style mechanism.

Also unlike ..., there is no mechanism in place to force the varargs
to appear last in the argument list. It will pick up the progress
through the arguments at the point it's reached, and will ignore any
later arguments. It would be possible to be more rigorous about this
by changing the callFrom templates, but the overhead in complexity
is probably not worth it.

Also, retrieving arguments through a VarArgs happens live, meaning at
the point that the argument is asked for. If the ThreadContext or
memory the argument lives in is modified before that point, the
retrieved value will reflect that modification and not what the
function was originally called with. Care should be taken so that this
doesn't cause corrupted arguments.

Finally, this mechansim (and the Guest ABI mechanism in general) is
complex and should have tests written for it. That should be possible
since ThreadContext is forward declared and so the test can say it
works however it wants or even ignore it completely. If that changes
in the future, we may need a mock ThreadContext implementation.

Jira Issue: https://gem5.atlassian.net/browse/GEM5-187

Change-Id: I37484b50a3e8c0d259d9590e32fecbb5f76670c1
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23195
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>

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