build: Add 'show-arches' target
build: Add 'show-arches' target

Add a new build target that displays all available
architectures and indicates the default:

```
make show-arches
```

Fixes #1009.

Signed-off-by: James O. D. Hunt <348fbaadfbada55ea105602bf5609e23ac1c4898@intel.com>

chavafg/runtime,clearcontainers/runtime,clearcontainers/runtime,devimc/runtime,devimc/runtime,chavafg/runtime