bpftool: Avoid using "?: " in generated code

"?:" is a GNU C extension, some environment has warning flags for its
use, or even prohibit it directly.  This patch avoid triggering these
problems by simply expand it to its full form, no functionality change.
Signed-off-by: Yucong Sun <fallentree@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210928184221.1545079-1-fallentree@fb.com

tools/bpf/bpftool/gen.c

@@ -803,7 +803,10 @@ static int do_skeleton(int argc, char **argv)
 			}						    \n\
 									    \n\
 			err = %1$s__create_skeleton(obj);		    \n\
-			err = err ?: bpf_object__open_skeleton(obj->skeleton, opts);\n\
+			if (err)					    \n\
+				goto err_out;				    \n\
+									    \n\
+			err = bpf_object__open_skeleton(obj->skeleton, opts);\n\
 			if (err)					    \n\
 				goto err_out;				    \n\
 									    \n\