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-gem5

@@ -133,6 +133,8 @@ https://github.com/cirosantilli/linux-kernel-module-cheat-regression#gem5-unit-t
                 [
                     # TODO reenable, broken, had enough of this.
                     # https://gem5.atlassian.net/browse/GEM5-357
+                    # https://gem5.atlassian.net/browse/GEM5-656
+                    # https://gem5.atlassian.net/browse/GEM5-778
                     #'SLICC_HTML=True', LF,
                 ] +
                 self.sh.add_newlines(targets) +