stdlib: Update the resources.json URL
stdlib: Update the resources.json URL

It is important this URL remains stable across releases. Pulling
directly from the Google Source git repo is not stable and may change
over time. This patch updates the URL to
https://resources.gem5.org/resources.json. As the gem5.org domain is
under the gem5 project's control, we can ensure this does not change.

In order to ensure this downloader remains stable, the reading of the
resources.json file has been updated to either accept plain-text of
base64.

Change-Id: I549fabb1525ee1df68cb1641c1bd82ea8bd32262
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/59269
Maintainer: Bobby Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5