The existing scheme for populating GPU vendor and device IDs fails
on macOS/ARM. If we find no PCI registry entries, look for a
AGXAccelerator entry, and (partially) populate that.

This is sufficient for Chrome to initialize hardware acceleration.
It is unknown how this will interact with multiple GPUs, and this
will likely need to be revisited.

Bug: chromium:1110421
Change-Id: I08069d7aecf45c83a1d2827cfccc4733c1835994
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2324939Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Lingfeng Yang <lfy@google.com>
Commit-Queue: ccameron <ccameron@chromium.org>