sim: Add a returnInto function to the SyscallDesc class.

sim: Add a returnInto function to the SyscallDesc class.

This method lets system call implementations return values into
ThreadContexts other than the one they were called from. That's useful
for, for instance, clone() which creates new ThreadContexts.

By making it a virtual function in the SyscallDesc, we can delegate the
actual implementation to the SyscallDescABI subclass which knows the
ABI and how to use it to set the return value.

Change-Id: I61c6e60e4c2a8863c885cd818e4ff053fc3312ee
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23503
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Gabe Black <gabeblack@google.com>