Check for export dynamic symbol options when doing symbol resolution in plugins.

Check for export dynamic symbol options when doing symbol resolution in plugins.

2017-09-20  Teresa Johnson  <tejohnson@google.com>
	* plugin.cc (is_visible_from_outside): Check for export dynamic symbol
        option and list.
	* testsuite/Makefile.am (plugin_test_12): New test.
	* testsuite/Makefile.in: Regenerate.
	* testsuite/export_dynamic_plugin.cc: New test source.
	* testsuite/plugin_test_12.sh: New test script.