Docker: start migrating to 20.04

Still failing with:

Traceback (most recent call last):
  File "util/cpt_upgrader.py", line 73, in <module>
    from six.moves import configparser
ImportError: No module named six.moves

Also fix some issues noticed:

- userland/c/atomic was not ignoring arch specific examples
- ./build would not stop on the first error, now it does
- add libhdf5-dev as a dependency of gem5

build-docker

@@ -45,7 +45,7 @@ See also: https://github.com/cirosantilli/linux-kernel-module-cheatTODO#ubuntu-g
             '-t',
             '-w', target_dir,
             '-v', '{}:{}'.format(kwargs['root_dir'], target_dir),
-            'ubuntu:18.04',
+            'ubuntu:20.04',
             'bash',
         ])
         self.sh.run_cmd([