ld/testsuite/ld-scripts/defined4.d: Don't xfail mmix-*-*.

A recent fix for the mmo input reader caused symbol-to-type mapping
(which is heuristic for the mmo format) to change (as in "corrected"),
and this test-case now passes.

Before that change, the comment was actually wrong: the symbol type
was mistakenly set to as B for uninitialized data, not T for text
section.