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

requirements.txt

@@ -1,3 +1,3 @@
 Cython==0.29.15
-china-dictatorship==0.0.52
+china-dictatorship==0.0.67
 pexpect==4.6.0