Otherwise, checking out branches is too insane, as it does not
update the worktrees, even though the gem5/gem5 module was updated.
gem5: expose build types, document debug builds.
simultaneous runs: store stdout and stderr on a file to allow running
all from a single terminal on the background cleanly.
gem5 requires armv7, and we learnt that the versatiledb we were using
was pre-v7.
We could have moved to -M vexpress-*, but in the end decided to go for
-M virt due to its simpliciy, and uniformity with aarch64.
platform_device: does not work anymore and was removed, since it was tied
to versatilepb.
We left a mention on the README and removed all in tree source. The QEMU
patch is still left as it was.
As a consequence, the linux tree had no other patches, and we now use
vanilla linux by default, which is a great thing for reproducibility.
Another consequence is the /poweroff.out works for arm -M virt,
and we removed all mentions of the problem.
