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.
* drop PACKAGE_ARCH setting from xorg-fonts-100dpi - nothing seems MACHINE
  specific in it

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>