libffi: Integrate testsuite with GCC testsuite

	* testsuite/lib/libffi.exp (load_gcc_lib): Load library from GCC
	testsuite.
	Load target-supports.exp and target-supports-dg.exp.
	(libffi-init): Use libraries in GCC build tree.
	(libffi_target_compile): Link with -shared-libgcc -lstdc++ for
	C++ sources.