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 <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>
Maintainer: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>

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