Move arm to virt armv7, unify arm QEMU and gem5 builds

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.

CONTRIBUTING.adoc

@@ -7,8 +7,8 @@ Testing that should be done for every functional patch.
 === Guest testing
 
 ....
-./run -a x86_64 -e '- lkmc_eval="/insrm.sh hello 5;wget -S google.com;poweroff;"'
-./run -a arm    -e '- lkmc_eval="/insrm.sh hello 5;wget -S google.com;poweroff;"'
+./run -a x86_64 -e '- lkmc_eval="/insrm.sh hello 5;/sbin/ifup -a;wget -S google.com;poweroff;"'
+./run -a arm    -e '- lkmc_eval="/insrm.sh hello 5;/sbin/ifup -a;wget -S google.com;poweroff;"'
 ....
 
 Should:
@@ -35,3 +35,8 @@ Shell 2:
 ....
 
 Should break GDB at `start_kernel`.
+
+Then proceed to do the following tests:
+
+* `/count.sh` and `b sys_write`
+* `insmod /timer.ko` and `b lkmc_timer_callback`