bpftool: Use __typeof__() instead of typeof() in BPF skeleton

When generated BPF skeleton header is included in C++ code base, some
compiler setups will emit warning about using language extensions due to
typeof() usage, resulting in something like:

error: extension used [-Werror,-Wlanguage-extension-token]
obj->struct_ops.empty_tcp_ca = (typeof(obj->struct_ops.empty_tcp_ca))
^

It looks like __typeof__() is a preferred way to do typeof() with better
C++ compatibility behavior, so switch to that. With __typeof__() we get
no such warning.

Fixes: c2a0257c1edf ("bpftool: Cast pointers for shadow types explicitly.")
Fixes: 00389c58ffe9 ("bpftool: Add support for subskeletons")
Signed-off-by: Andrii Nakryiko <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Kui-Feng Lee <[e-mail]>
Acked-by: Quentin Monnet <[e-mail]>
Acked-by: John Fastabend <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/bpf/bpftool/gen.c

···
 			 */
 			needs_typeof = btf_is_array(var) || btf_is_ptr_to_func_proto(btf, var);
 			if (needs_typeof)
-				printf("typeof(");
+				printf("__typeof__(");
 
 			err = btf_dump__emit_type_decl(d, var_type_id, &opts);
 			if (err)
···
 			continue;
 		codegen("\
 			\n\
-				obj->struct_ops.%1$s = (typeof(obj->struct_ops.%1$s))\n\
+				obj->struct_ops.%1$s = (__typeof__(obj->struct_ops.%1$s))\n\
 					bpf_map__initial_value(obj->maps.%1$s, NULL);\n\
 			\n\
 			", ident);