Generate the entry points for NV_framebuffer_blit.

Bug: angleproject:5474
Bug: chromium:1157057
Change-Id: Iadffa24ce7368d8ab6c4bf9d5b6c016276e762f5
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2602242Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>