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.

common

@@ -24,7 +24,7 @@ set_common_vars() {
   gem5="${2:-false}"
   buildroot_dir="${root_dir}/buildroot"
   arch_dir="$arch"
-  if "$gem5" && [ ! "$arch" = aarch64 ]; then
+  if "$gem5" && [ "$arch" = x86_64 ]; then
     arch_dir="${arch}-gem5"
   fi
   out_arch_dir="${out_dir}/${arch_dir}"