sim: Convert most of the common syscalls to use the guest ABI.

sim: Convert most of the common syscalls to use the guest ABI.

A couple tricky instances were left alone for now, specifically one
place where the size of the argument is specified explicitly (the
default ABI always leaves off the size), and two places where the
arguments are variadic which is not yet supported.

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

Change-Id: I2ec19bea77d329de3575e17e9f7209f4fd871aa6
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23193
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>