scons: conditional use of new RPATH inclusion

On OSX, clang doesn’t support the -z option.
This patch resolve the compiler error produced on MacOS platform.

Change-Id: Idfe69c30fe40add97d16d0f2e25e598b30d26a9d
Reviewed-on: https://gem5-review.googlesource.com/c/16649
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>