package/kvm-unit-tests: introduce _ARCH_SUPPORTS

package/kvm-unit-tests: introduce _ARCH_SUPPORTS

Move all architecture options to their own symbol, so that it is easier
to add more variants in the future.

The dependency on cross-gcc >= 4.5 is only valid for i386, as we use the
host gcc for x86_64. Adapt the dependency accordingly.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Cyril Bur <cyrilbur@gmail.com>
[Thomas:
 - add comment about host gcc >= 4.5 dependency on x86-64
 - remove bogus "depends on ... if ..." construct.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>