ARM: 8534/1: virt: fix hyp-stub build for pre-ARMv7 CPUs

ARMv6 CPUs do not have virtualisation extensions, but hyp-stub.S is
still included into the image to keep it generic. In order to use ARMv7
instructions during HYP initialisation, add -march=armv7-a flag to
hyp-stub's build.

On an ARMv6 CPU, __hyp_stub_install returns as soon as it detects that
the mode isn't HYP, so we will never reach those instructions.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>