mulitlib.bbclass: Ensure correct value of ALL_MULTILIB_PACKAGE_ARCHS is preserved
The value of ALL_MULTILIB_PACKAGE_ARCHS needs to be consistent both
in multilib extended recipes and in normal context. If this isn't the
case it can lead to inconsistent configuration files at a minimum.
This patch ensures the value is preserved during the class extension code
since computing it after that point is hard.
[YOCTO #2290]
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
