systemc: Add a filter-file option.

systemc: Add a filter-file option.

The --filter option is very useful, but can get to be long and complex
and a bit too cumbersome to use from the command line. This change
adds a --filter-file option which is mutually exclusive with --filter
and which reads the filter expression from a file instead of accepting
it on the command line.

Change-Id: I381c92ddf0d9fe62acd20432fa4868e2121405b8
Reviewed-on: https://gem5-review.googlesource.com/11257
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>