scons: Group Source-s based on what SConscript included them.

scons: Group Source-s based on what SConscript included them.

The groups won't be perfectly balanced or optimally planned, but this
requires no thought and breaks the object files down into a reasonable
number of reasonably sized groups.

Change-Id: I6542fc807aaf356a9be751093f68e2e29f0b1586
Reviewed-on: https://gem5-review.googlesource.com/2946
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>