sim: Add an option to suppress the return value in invokeSimcall.

sim: Add an option to suppress the return value in invokeSimcall.

Sometimes when using the GuestABI mechanism, gem5 wants to know that a
function was called and with what arguments to do its own processing,
but doesn't want to return its own value since it will still let the
simulated system execute its own function. There are also situations
where gem5 wants to return a value, but not through the normal
mechanism. That happens when, for instance, a gem5 op is triggered by a
memory access, and that access is what should return the value, not a
particular fixed register.

This option is a template parameter rather than a function argument so
that if it's not going to be used, no "Return" type needs to be defined
since it's not present at all in the chain of functions invokeSimcall
expands to.

This will also make it easier to reuse generic ABIs in those situations
without having to make custom wrappers.

Change-Id: I969e78495c8f4e73f4de1a3dfb4d74c9b30f5af5
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28288
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>