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

path_properties.py

@@ -690,7 +690,18 @@ path_properties_tuples = (
                         # This has complex failure modes, too hard to assert.
                         'smash_stack.c': {'skip_run_unclassified': True},
                         'std_atomic.c': {'baremetal': False},
-                        'atomic': {'baremetal': False},
+                        'atomic': (
+                            {
+                                'test_run_args': {'cpus': 3},
+                            },
+                            {
+                                'aarch64_add.c': {'allowed_archs': {'aarch64'}},
+                                'aarch64_ldadd.c': {'allowed_archs': {'aarch64'}},
+                                'aarch64_ldaxr_stlxr.c': {'allowed_archs': {'aarch64'}},
+                                'x86_64_inc.c': {'allowed_archs': {'x86_64'}},
+                                'x86_64_lock_inc.c': {'allowed_archs': {'x86_64'}},
+                            },
+                        ),
                         # Wrapper not defined by newlib.
                         'timespec_get.c': {'baremetal': False},
                     }