mulitlib.bbclass: Ensure correct value of ALL_MULTILIB_PACKAGE_ARCHS is preserved

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]

(From OE-Core rev: 529bc145152bb36d9696226b93729377a3b6a240)

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