testsuite: Fix g++.dg/ext/attr-copy-2.C for default_packed targets

For targets where the byte-wise structure element layout is
the same as for attribute-packed, you get a warning when
that's specified.  Thus, expect a warning for such targets.
Note the exclusion of bitfields.

	* g++.dg/ext/attr-copy-2.C: Fix for default_packed targets.