scons: Set the partial linking group for EXTRAS dirs.

Partial linking heuristically links together files in the same
directory by setting a special automatic tag. That tag needs to also
be maintained when scanning EXTRAS dirs so that they don't all get
lumped in with the last normal directory that was processed.

Change-Id: I2408ea0a1eeffcf6d9994c36415a35760b225b17
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23300
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/SConscript

@@ -606,6 +606,7 @@ for extra_dir in extras_dir_list:
 
         if 'SConscript' in files:
             build_dir = joinpath(env['BUILDDIR'], root[prefix_len:])
+            Source.set_group(build_dir)
             SConscript(joinpath(root, 'SConscript'), variant_dir=build_dir)
 
 for opt in export_vars: