testsuite: Handle C++29 in libstdc++ std module exports plugin

Similar thing to the last, we also want to verify std.cc module exports
in C++29 mode.

2026-08-06  Jakub Jelinek  <jakub@redhat.com>

	* g++.dg/plugin/plugin.exp (plugin_test_list): Add
	std-module-exports-c++29.C.
	* g++.dg/plugin/std_module_exports_plugin.cc (whitelist_std): Add
	cxx29 enumerator, change value of all.
	(plugin_dump_decl): Handle C++29.
	* g++.dg/plugin/std-module-exports-c++29.C: New test.
3 files changed