Allow static/shared modules to be disabled per-arch

libz has a case where the static and shared versions of the device
library are supported, but the host shared library must not be defined,
since it would conflict with the system provided version. There's a
separate module 'libz-host' that provides the host shared library
implementation.

So extend the 'static'/'shared' properties to include 'enabled', which
uses arch_variant so that it can be mutated per-arch. It cannot override
a top level 'enabled: false'.

Change-Id: Ia0f1ff31ad77dc4cb148c531b70158e3245031d5