arm: Add a unit test for some aspects of the aapcs64 ABI.

arm: Add a unit test for some aspects of the aapcs64 ABI.

This test covers the templates which attempt to classify types, but not the
actual gathering of arguments of distribution of return values. As before, we
can't really use standard C++ to accurately test for HFAs and HVAs, so we stick
with approximating them by detecting arrays of the right types.

For example, I think technically we should also accept a struct with only 4
float members, but c++ templates aren't able to match against types in that way
as far as I know.

Change-Id: I1d7756a964a86c0c5ea13e068a5fc74603e14e30
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28268
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>