packagegroup/allarch: Convert to use allarch class

packagegroup/allarch: Convert to use allarch class

Currently there is some odd behaviour of the packagegroup class in relation
to sstate since it sets PACKAGE_ARCH = "all" but does not use the allarch class
leading to it being undetected by sstate.

Previously it was not possible to use allarch as the recipe couldn't "undo"
settings made by the allarch class. Since this no longer happens when
PACKAGE_ARCH != all, we can use the allarch class.

This patch also fixes up one case we need to preserve TRANSLATED_TARGET_ARCH
and ensures sstate only assumes allarch when PACKAGE_ARCH is "all".

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>