Update libEGL.cpp with KHR version of sync entry points
Update libEGL.def with sync KHR entry point names

Bug: angleproject:4383
Change-Id: Ibda23a9e2ba6c873384238d492f0895b370fd6af
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2212892Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>