scons: allow embedding arbitrary blobs into the gem5 executable

The initial motivation for this is to embed the GDB XML target
description files into the executable.

Change-Id: I721e8dd37119d8e6eb376d7e9050b1094282bacc
Reviewed-on: https://gem5-review.googlesource.com/c/15136
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>