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>

src/SConscript

@@ -359,6 +359,7 @@ for root, dirs, files in os.walk(base_dir, topdown=True):
 
     if 'SConscript' in files:
         build_dir = joinpath(env['BUILDDIR'], root[len(base_dir) + 1:])
+        Source.set_group(build_dir)
         SConscript(joinpath(root, 'SConscript'), variant_dir=build_dir)
 
 for extra_dir in extras_dir_list: