Change RPM builds architecture to 'x86_64'

[da2x]

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).

Test Plan:

1. Build a release, install on Fedoara with dnf install package.rpm. Package name should end with x86_64.rpm. Package should install with no errors.
2. Build a new release (higher version number). Upgrade on Fedoara with dnf install package.rpm. Should update with no errors.
3. Debian/Ubuntu package names and installation should be unchanged.

Core issue:
Incorrect documentation from upstream. Submitted correction to their documentation.

Commit message:
Red Hat/RPM universe expects 'x86_64' whereas Debian expects 'amd64'.

Also expands documentation in some areas.

This closes #3774.

[diracdeltas]

seems good, thanks