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.

kernel_module/user/README.adoc

@@ -20,7 +20,9 @@ These programs can also be compiled and used on host.
 .... link:init_dev_kmsg.c[]
 .. link:uio_read.c[]
 .. link:rand_check.c[]
-.. link:rdtsc.c[]
+.. x86_64
+... link:rdtsc.c[]
+... link:ring0.c[]
 . Module tests
 .. link:anonymous_inode.c[]
 .. link:poll.c[]