multilib: fix SSTATE_ARCHS for multilib usage

multilib: fix SSTATE_ARCHS for multilib usage

When building multilibs, we need to inject the multilib sstate pkgarch
into SSTATE_ARCHS so the list forms a complete search path. Add a tweak
to do this.

PACKAGE_ARCH defaults to TUNE_PKGARCH so this is equivalent and just
guards against recipes changing the value which may have other unwanted
side effects.

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