MFC r345316, r345317, r345858:

r345316: Register ARM64 EC2 AMIs as being for the ARM64 architecture.

r345317: Don't install amazon-ssm-agent package into ARM64 AMIs.

r345858: Add support for cross-building cloudware images.

With these changes it is possible to build ARM64 EC2 AMIs on stable/12 --
said images do not *work* yet, however.