[patch] Adjust target selectors for some libstdc++ tests

[Jonathan Wakely]

This marks a test that is xfail for darwin as also xfail for
dragonfly. I'm surprised it fails on darwin and dragonfly but not
freebsd, but Gerald's testresults don't show it failing.

Also fix a couple of tests which were missing target selectors on the
{ dg-do compile } directive, so the tests ran everywhere but failed on
any platform that wasn't listed as a target in the { dg-options }
directive.

Tested x86_64-unknown-linux-gnu and x86_64-unknown-dragonfly3.6,
committed to trunk. 

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