concretizer bugfix: all variants need possible values

Variants like `cpu_target` on `openblas` don't have defineed values, but
they have a default.  Ensure that the default is always a possible value
for the solver.

lib/spack/spack/solver/asp.py

@@ -354,6 +354,10 @@ def pkg_rules(self, pkg):
                     union.update(s)
                 values = union
 
+            # make sure that every variant has at least one posisble value
+            if not values:
+                values = [variant.default]
+
             for value in sorted(values):
                 self.fact(fn.variant_possible_value(pkg.name, name, value))