This steps towards full auto-generation of EGL entry point files.

Bug: angleproject:2621
Change-Id: I629447bb6754ee1e87bef020de21b28e37e20eaf
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2552774
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>