Also parallelize some of the more lengthy file generation steps
Bug: dawn:1479
Change-Id: I7674fca4958e4d9948e287008916c4b0d33e1ca1
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/97022
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>