kvm-unit-tests: only enable for ARM on Cortex-A{7,12,15,17}

kvm-unit-tests: only enable for ARM on Cortex-A{7,12,15,17}

The library uses the hypervisor instruction HVC, which is only
available on ARMv7-A with Hypervisor extensions. This is limited to
Cortex-A{7,12,15,17}.

In addition the build unconditionally passes -marm so it also needs
ARM instructions to be available, but that is already implied by the
CPU selection.

Implicitly fixes
http://autobuild.buildroot.net/results/53d109fd9055fd20387bb857aced5f89cf3086fd
though it is still not clear why ld doesn't accept the -Ttext=...
option there.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[Thomas: A12 and A17 also have the virtualization extensions.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>