Remove struct_size_helper from LVL build. This was causing
significant problems on the Chromium CQ.

Also fix the warning of duplicate definitions in reg.py by
staging scripts to a temporary folder so we can use a fixed
version.

Bug: angleproject:2482
Bug: chromium:833736
Change-Id: Ide7202fe14bf6c83419f19bc07113df23d51807a
Reviewed-on: https://chromium-review.googlesource.com/1014902
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>