build: use TARGET_SUPPORTS_32_BIT_APPS and TARGET_SUPPORTS_64_BIT_APPS

Use TARGET_SUPPORTS_32_BIT_APPS and TARGET_SUPPORTS_64_BIT_APPS to
determine which native libraries to build for an app.  If
both are set, it will use 64-bit unless TARGET_PREFER_32_BIT is set.
If only one is set, it will only build apps that work on that
architecture.  If neither is set it will fall back to only building
32-bit apps.

On existing 32-bit devices neither variable will be set, and the
build system will continue to build 32-bit apps.

Once a device has support for a 64-bit runtime, the same logic
that selects the dual runtimes should set TARGET_SUPPORTS_32_BIT_APPS
and TARGET_SUPPORTS_64_BIT_APPS, and packages will be built for
the preferred arch, or fall back to the non-preferrred arch if
necessary.

For testing, a device may set TARGET_SUPPORTS_64_BIT_APPS without
TARGET_SUPPORTS_32_BIT_APPS to produce only 64-bit apps.

Change-Id: I5b5e23f15602c3cf9bd96791971208a85492c7a3