Refactor machine specific private config settings into GDK::Machine

What does this Merge Request do and why?

This change moves machine specific private configurations into GDK::Machine methods:

• config.__platform => GDK::Machine.platform
• config.__architecture => GDK::Machine.architecture
• config.__platform_linux => GDK::Machine.linux?
• config.__platform_darwin => GDK::Machine.macos?
• config.__platform_supported => GDK::Machine.supported?

Adds the following methods:

• GDK::Machine.x86_64?
• GDK::Machine.arm64?

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.