misc,tests: Updated and generalized the resource url for tests

As we are now going to maintain different bundles of resources for each
gem5 release, the resources have been archived to
http://dist.gem5.org/dist/current for gem5 19. The development branch
will use http://dist.gem5.org/dist/develop going forward. New releases
will follow the format http://dist.gem5.org/dist/{VERSION}.

This patch makes the resources url a command-line parameter, set to the
"correct" url by default. This will be updated to the correct, archived,
version subdirectory upon release of a new gem5 version. E.g.:
http://dist.gem5.org/dist/v20 for the gem5 20 release.

Some Make files have been cleaned up to no longer fetch and push to
remote locations. As gem5-resources is implemented, sources will be
moved to the gem5-resources resository and compiled binaries, etc.
pulled from our Google Cloud bucket from the tests themselves.

Change-Id: Ia16c496be3a60283ecc431ffaa5b059e1932b526
Issue-on: https://gem5.atlassian.net/browse/GEM5-431
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27987
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

ext/testlib/config.py

@@ -220,6 +220,7 @@ def define_defaults(defaults):
                                                       os.pardir))
     defaults.result_path = os.path.join(os.getcwd(), '.testing-results')
     defaults.list_only_failed = False
+    defaults.resource_url = 'http://dist.gem5.org/dist/develop'
 
 def define_constants(constants):
     '''
@@ -579,6 +580,12 @@ def define_common_args(config):
             default=None,
             help='Path where binaries are stored (downloaded if not present)'
         ),
+        Argument(
+            '--resource-url',
+            action='store',
+            default=config._defaults.resource_url,
+            help='The URL where the resources reside.'
+        ),
 
     ]