Partial support for GS was added by jaedon1.lee@samsung.com already.
This change enables the extension and adds suppressions for GLES3.1 dEQP
tests.  Suppressions for KHR GLES3.2 will be updated separately as
currently GS shaders in "version 320" fail validation.

Bug: angleproject:3571
Change-Id: Ic6ed65554937fd0cce2d0f6ea0ac2eb7e6ed9f10
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2561445Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>