arm: build with virtualisation support

arm: build with virtualisation support

Most armv7-compatible toolchains still need an additional flag to
recognise instructions such as ERET or an MSR to banked registers.
This patch allows to includes the virt flag in files that require it.
It also forces the hypervisor image to only use the ARM instruction
set.
Support for Thumb2 hypervisor and kernel will be added later.
Guests should still be able to run in Thumb2, as long as they allow
to be entered in ARM mode.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>