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

Dockerfile

@@ -1,5 +1,5 @@
 # https://cirosantilli.com/linux-kernel-module-cheat#docker
-FROM ubuntu:18.04
+FROM ubuntu:20.04
 COPY setup /
 RUN /setup -y
 CMD bash