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/README.adoc

@@ -43,11 +43,11 @@
 .. link:virt_to_phys.c[]
 . Utilities
 .. link:kstrto.c[]
-. Misc
-.. link:ring0.c[]
-. ARM
-.. link:pmccntr.c[]
+. Arch
+.. x86
+... link:ring0.c[]
+.. ARM
+... link:pmccntr.c[]
 . Hardware device drivers
 .. link:pci_min.c[]
 .. link:pci.c[]
-.. link:platform_device.c[]