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.