Output builds to top-level obj and bin folder.

Change-Id: I1444a3d15977c2617866f293e0ac0ccbd253d878
Reviewed-on: https://swiftshader-review.googlesource.com/5353
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>