scons: Use Dir().Dir() and not os.path to extend CPPPATH.
scons: Use Dir().Dir() and not os.path to extend CPPPATH.

Since we're already working with Dir nodes, they can figure out
appending to a path themselves without using os.path.join.

Change-Id: Ib46946d7ec181dbbf443f957f23196eb0fd7f6b5
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48127
Reviewed-by: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Maintainer: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5