testsuite: Skip some gcc.dg/plugin tests for default_packed targets

Avoid unweildy structure-layout-specific message-matching
expressions by exluding targets that lay out structures as
if they had been specified with __attribute__ ((__packed__)),
for tests where multiple messages depend on the structure
layout.

It's arguably a judgement call whether to skip some of these
tests or add multiple lines of matches depending on the
layout of structures.

	* gcc.dg/plugin/infoleak-2.c,
	gcc.dg/plugin/infoleak-CVE-2011-1078-1.c,
	gcc.dg/plugin/infoleak-CVE-2011-1078-2.c,
	gcc.dg/plugin/infoleak-CVE-2017-18549-1.c,
	gcc.dg/plugin/infoleak-CVE-2017-18550-1.c,
	gcc.dg/plugin/infoleak-antipatterns-1.c,
	gcc.dg/plugin/infoleak-fixit-1.c: Skip for default_packed targets.