multilib.bbclass: Reduce ALTERNATIVE_PRIORITY for extended recipes

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

$ bitbake core-image-minimal
update-alternatives: libtool has multiple providers with the same priority,
please check
/path/to/rootfs/usr/lib/opkg/alternatives/libtool for details

Both libtool and lib32-libtool have the same priority (as they're the same
recipe), so update-alternatives won't deterministically pick a provider. This
means you could end up with an image using a 32-bit pkgconfig and 64-bit
libtool, for example.

Make extended recipes reduce priority by 1 (or 2, 3 ... when there are multiple
variants in MULTILIB_VARIANTS) to fix the problem.

[YOCTO #13418]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>