scons: Put Source objects in groups and partially link them.

scons: Put Source objects in groups and partially link them.

The groups will be linked together into intermediate partially linked
object files. Right now the hierarchy is assumed to be flat, but with some
effort it could be extended to allow truly hierarchical linking.

Change-Id: I77b77710554e5f05e8b00720a0170afaf4afac2d
Reviewed-on: https://gem5-review.googlesource.com/2945
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>