sim: Make SyscallReturn handle extra/"pseudo" return registers.

sim: Make SyscallReturn handle extra/"pseudo" return registers.

Avoid special casing them in the system calls themselves.

Change-Id: I735f8e6fdff164c66e3f1386aed3fc9b107ea45f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23440
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>