sim: Add a wrapper/subclass for SyscallDesc which uses GuestABI.

sim: Add a wrapper/subclass for SyscallDesc which uses GuestABI.

This will let system call implementations take arguments naturally,
and centrally defined, potentially complex, and ISA/context specific
mechanisms will automatically gather the arguments and store any
result.

Jira Issue: https://gem5.atlassian.net/browse/GEM5-187

Change-Id: I68d265e0bab5de372ba975e4c7e9bb2d968c80af
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23172
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>