Fix special 5.6. associations for $, $\ as they are no SVs there

Fixes one part of #306 on 5.6.

There still exists a refcnt problem on 5.6 (invalid free) with t/issue256.t
And the remaining #306 regressions with t/CORE/io/print.t, t/CORE/op/tiehandle.t
and t/CORE/op/smartkve.t