We shouldn't need the extra loader defines config variable. The defines
we need should probably be able to be located in ANGLE entirely.

Bug: angleproject:3320
Bug: angleproject:3876
Change-Id: I78f449f44c1709b6d683930f9b9ea3d80016272e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1775462Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>