Warn for missing BuildArchitecture during Layer build

[bentvelj]

NOTE: I've moved the check for BuildArchitecture ∈ CompatibleArchitectures from PR #6322 (related issue: #6110) into the Layer build, so that it's not exclusive to Makefile Layer builds.

To summarize:

• All Layer builds will warn if BuildArchitecture is missing
• All Layer builds will warn if BuildArchitecture is not in CompatibleArchitectures
• Only Makefile Layer builds will check and warn on invalid (not x86_64 or arm64) BuildArchitecture. Other BuildMethods will verify this in LambdaBuilders already, and fail on bad architecture.

Which issue(s) does this change fix?

#3747

Why is this change necessary?

Customer is confused as to why the build does not use their CompatibleArchitectures when they omit BuildArchitecture. It has already been decided to default to the case of x86_64 in the case of missing BuildArchitecture, so we can warn the customer to add it if it is missing.

How does it address the issue?

Adds a warning to Layer builds when BuildArchitecture is missing, and that x86_64 is being used as a default.
Also warns if the BuildArchitecture specified is not in CompatibleArchitectures.

What side effects does this change have?

N/A

Mandatory Checklist

PRs will only be reviewed after checklist is complete

• Add input/output type hints to new functions/methods
• Write design document if needed (Do I need to write a design document?)
• Write/update unit tests
• Write/update integration tests
• Write/update functional tests if needed
• make pr passes
• make update-reproducible-reqs if dependencies were changed
• Write documentation

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[sidhujus]

LGTM!