systemc: Change the test binaries to use rpath.

systemc: Change the test binaries to use rpath.

Use rpath to link the gem5 dynamic library into the systemc test
binaries so that they don't have to be run from a particular directory
to resolve all their linking dependencies.

Change-Id: I66b18c23ae6bbf32a959022f8789fc8bdd3a6c6b
Reviewed-on: https://gem5-review.googlesource.com/12270
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>