Add support for small ARM machine types

Use Case

https://aws.amazon.com/about-aws/whats-new/2021/09/aws-codebuild-small-arm-machine-type/

Proposed Solution

add SMALL machine type to validation check for ArmBuildImage

• 👋 I may be able to implement this feature request
• ⚠️ This feature might incur a breaking change

This is a 🚀 Feature Request