Please provide go<->appc arch label translators

[lucab]

appc/spec defines its own set of arch labels which don't always align with architecture labels used by other frameworks, including Golang ones (which also encompass an GOARCH/GOARM tuple for arm).

This has caused some pain in the past for projects using appc, eg:

• Perform proper arch-label translation rkt/rkt#2369
• lib: wrong arch label values on non-amd64 architectures docker2aci#195
• docker i386 images can't be converted (arch 386 v. i386) docker2aci#145

appc/spec should start providing a couple of translators to help performing this go<->appc mapping.

/cc @s-urbaniak who recently hit this again.