Uses a GN copy rule to duplicate the loader. Also updates volk to
load using the new custom loader. Once both are in place we can
go back and remove the copy.

Bug: chromium:1219969
Change-Id: I8c48d168a842539f7cdba1ebfdaf3b08c3e1990d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2982499
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>