sim: Add GenericSyscallABI structs which can be used by the ISAs.

sim: Add GenericSyscallABI structs which can be used by the ISAs.

It's very common for system call arguments to be passed in a sequence
of registers, one argument per register. To avoid having that
implementation repeated over and over across the various ISAs and OSes,
these partial ABI implementations provide that mechanism they can just
pull in. They would need to define the sequence of registers to use,
and these would take care of the rest.

Unlike the temporary DefaultSyscallABI which defers to the Process
classes, these read registers from the ThreadContext directly.

Change-Id: Ic72eb8d784ecf4711b5eec76d958a87c70850fce
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23441
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>