Implement some missing entry points, including sync objects.

Bug: b/150458446
Bug: angleproject:3662
Change-Id: I0e9e7926e1a13bd9b0e83900ea19d2fbef9a284f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2220312Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Cody Northrop <cnorthrop@google.com>