staging.bbclass: extend_recipe_sysroot(): search multilib manifest

staging.bbclass: extend_recipe_sysroot(): search multilib manifest

Fixed:
MACHINE = "qemux86-64"
require conf/multilib.conf
MULTILIBS = "multilib:lib32"
DEFAULTTUNE_virtclass-multilib-lib32 = "core2-32"

$ bitbake lib32-wrlinux-image-glibc-small
WARNING: lib32-wrlinux-image-glibc-small-1.0-r1 do_rootfs: Manifest for lib32-qemuwrapper-cross not found, searched manifests:
/path/to/tmp/sstate-control/manifest-qemux86_64-lib32-qemuwrapper-cross.populate_sysroot
/path/to/tmp/sstate-control/manifest-core2-64-lib32-qemuwrapper-cross.populate_sysroot
/path/to/tmp/sstate-control/manifest-x86_64-lib32-qemuwrapper-cross.populate_sysroot
/path/to/tmp/sstate-control/manifest-allarch-lib32-qemuwrapper-cross.populate_sysroot

It was because image recipes' multilib PACKAGE_EXTRA_ARCHS are not changed by
mutlilib.bbclass, so extend_recipe_sysroot() didn't know anything about
mutlilib's PACKAGE_EXTRA_ARCHS, now fix it.

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