packagegroup-*: set PACKAGE_ARCH before packagegroup inherit

The required order was changed in OE-Core commit:

,----
| allarch: Add warning about packagegroup
|
| Since we want to start including this class conditionally, detect cases
| where packagegroup files use the old ordering and inform the user they
| need to update this.
`----

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>