EGL: Auto-generate "libEGL" export cpp file. 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>