multilib_global: expand multilib pref values properly

This ensures that in cases where the preference value changes when the
multilib override is applied, we correctly expand it in that context.

For example, for `PREFERRED_PROVIDER_${TARGET_PREFIX}gcc
= "gcc-external-cross-${TARGET_ARCH}"`, when it sets the prefixed version of
this, we want TARGET_ARCH expanded with the multilib applied, otherwise the
arch suffix will be incorrect for that context.

We ran into this trying to use preferences in meta-sourcery along with
multilibs. We worked around it there via PNBLACKLIST, but this fix should
still go into the core.

(From OE-Core rev: 4d208ebacb3a5d189998ac9be6d1a454c45aa975)

Signed-off-by: Christopher Larson <kergoth@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/multilib_global.bbclass

@@ -84,7 +84,7 @@ def preferred_ml_updates(d):
             # implement alternative multilib name
             newname = localdata.expand("PREFERRED_PROVIDER_" + virt + p + "-" + pkg)
             if not d.getVar(newname, False):
-                d.setVar(newname, newval)
+                d.setVar(newname, localdata.expand(newval))
         # Avoid future variable key expansion
         provexp = d.expand(prov)
         if prov != provexp and d.getVar(prov, False):