sim: Add a suppression mechanism to the SyscallReturn class.

sim: Add a suppression mechanism to the SyscallReturn class.

It makes more sense to specify whether something should be returned
based on the return, not intrinsically on the syscall. This is
especially true in cases like execve where the expected behavior
is not constant.

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

Change-Id: I95b53b6d69445c7a04c0049fbb0f439238d971e8
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23166
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>