python-numpy: introduce ARCH_SUPPORTS hidden variable

Some packages selects python-numpy. This package has a some CPU specific
code. To simplify and avoid duplicate dependencies this patch introduce an
hidden ARCH_SUPPORTS variable.

[Thomas: fix the definition of the new option to make sure its value
is actually 'y' when a supported architecture is used.]

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>