This implements glGenSemaphoresEXT, glDeleteSemaphoresEXT, and
glIsSemaphoreEXT. It's not possible to do anything useful with them yet.

Bug: angleproject:3289

Change-Id: I20ad90dbcd3fc573a4650c8531d6e1b8ccf7ca9b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1623811
Commit-Queue: Michael Spang <spang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>