This adds entrypoints for the Linux variants of GL_EXT_memory_object &
GL_EXT_semaphore.

Bug: angleproject:3289

Change-Id: I40de40f27aa82cd9479d5913dac0a7493919bb8f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1552026
Commit-Queue: Michael Spang <spang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>