This reworks much of the Makefile and, to a lesser extent, the
Dockerfiles for building the CNI plugins for other platforms.

Additional clean up work is included as well as this was combed
through to support these builds.

Including:

- ARCH resolver to identify the ARCH (GOARCH styled) to pass around
- Updated ARCH and GOARCH references throughout to support
  against other ARCHes
- Make base container build appropriately for ARCH
  - Docker explicit multiarch references in targets and Dockerfiles
  - Updated related container images to allow testing where possible
- Eliminate ./tmp usage with tar unpacking from a pipe
- Clean up and consistent call sites all around
   - Reconciled image naming
   - Updated stale golang images references
- Added docstrings for reference

Though the tests don't require CGO with cross-compilation support, the
tests do still require support to exec the golang test binaries.