Add generation of libGLESv2.cpp and libGLESv2.def to generate_entry_points.py

Bug: angleproject:2476
Change-Id: I86c7fb31f73ccbbbc32b28acca179b11527dff9e
Reviewed-on: https://chromium-review.googlesource.com/1040647
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>