sim: Generalize the GuestABI Result::allocate() mechanism.

sim: Generalize the GuestABI Result::allocate() mechanism.

This change generalizes the GuestABI Result template family's
allocate() mechanism so that it can also be used with arguments. This
is useful in cases like the 32 bit ARM ISA which says that variadic
functions treat their floating point arguments differently than
non-variadic functions do. This mechanism will give the ABI a chance
to detect that the function is variadic (by checking for VarArgs) and
then to behave differently in that case.

This also makes the GuestABI mechanism a little more regular by
supporting allocate() on both arguments and return values instead of
just return values.

Change-Id: I67050a0ad7ccf15ee75b800dca4eae4fdc0e564e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24103
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Gabe Black <gabeblack@google.com>