Relax regexp in gdb.rust/unsized.exp

With nightly rustc, gdb.rust/unsized.exp fails:

    (gdb) ptype *us
    Structure has no component named operator*.

rustc changed to emit a bit more debug info for unsized types.
Because the original test is just to make sure that ptype of an
unsized array looks right, this patch relaxes the regexp and changes
the expression.  I think this keeps the original test meaning, but
also works with nightly.  I also tested stable and 1.48.