Attempt to fix libsupc++ support.

Recent versions of libstdc++ appear to have lost the clean layering
between libstdc++ and libsupc++, so use the high-level APIs and some
pointer evilness to fudge the low-level ones...

objcxx_eh.cc

@@ -22,8 +22,19 @@ extern "C"
 void __cxa_throw(void *thrown_exception, std::type_info *tinfo,
                  void (*dest)(void *));
 
+#ifdef __GLIBCXX__
+#include <exception>
+static void *__cxa_current_primary_exception()
+{
+	std::exception_ptr p = std::current_exception();
+	void *obj = *(void**)&p;
+	*(void**)&p = nullptr;
+	return obj;
+}
+#else
 extern "C"
 void *__cxa_current_primary_exception();
+#endif
 
 using namespace __cxxabiv1;