Opengl32.lib/opengl32.dll are not available on Windows ARM64. Disable building
glmard2_wgl which depends them.

Bug: chromium:941621

Change-Id: Iddae231764c6135ef1a74c294da1393f0fe51eeb
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1522353Reviewed-by: Jamie Madill <jmadill@chromium.org>