sim: Make the syscalls use the SyscallReturn suppression mechanism.

sim: Make the syscalls use the SyscallReturn suppression mechanism.

This, among other things, prevents them from needing to toggle global
flags in the syscall desc table to control local behavior.

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

Change-Id: Idcef23766084f10d5205721b54a6768a850f7eb9
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23167
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>