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

common.py

@@ -1453,7 +1453,7 @@ lunch aosp_{}-eng
         '''
         Run timed_main across all selected archs and emulators.
 
-        :return: if any of the timed_mains exits non-zero and non-null,
+        :return: if any of the timed_mains exits non-zero and non-None,
                  return that. Otherwise, return 0.
         '''
         env = kwargs.copy()