staging.bbclass: staging_populate_sysroot_dir(): fix for multilib

staging.bbclass: staging_populate_sysroot_dir(): fix for multilib

Fixed:
MACHINE = "qemux86-64"
require conf/multilib.conf
MULTILIBS = "multilib:lib32"
DEFAULTTUNE_virtclass-multilib-lib32 = "x86"

$ bitbake core-image-sato lib32-core-image-sato
$ bitbake build-sysroots

All lib32 manifests which had been built should be installed, but only a few
such as qemux86_64-lib32-base-files were installed (it was installed because
MACHINE_ARCH was still qemux86-64 when multilib), but others such as
x86-lib32-zlib were not installed, this was incorrect. For multilib builds,
fix-up overrides to prepend :virtclass-multilib- and then again append
additional multilib arches from the PACKAGE_EXTRA_ARCHS list if needed can fix
the problem.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>