bpf: Add typecast to raw_tracepoints to help BTF generation

When pahole converts dwarf to btf it emits only used types.
Wrap existing __bpf_trace_##template() function into
btf_trace_##template typedef and use it in type cast to
make gcc emits this type into dwarf. Then pahole will convert it to btf.
The "btf_trace_" prefix will be used to identify BTF enabled raw tracepoints.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20191016032505.2089704-2-ast@kernel.org

include/trace/bpf_probe.h

@@ -74,11 +74,12 @@ static inline void bpf_test_probe_##call(void)				\
 {									\
 	check_trace_callback_type_##call(__bpf_trace_##template);	\
 }									\
+typedef void (*btf_trace_##call)(void *__data, proto);			\
 static struct bpf_raw_event_map	__used					\
 	__attribute__((section("__bpf_raw_tp_map")))			\
 __bpf_trace_tp_map_##call = {						\
 	.tp		= &__tracepoint_##call,				\
-	.bpf_func	= (void *)__bpf_trace_##template,		\
+	.bpf_func	= (void *)(btf_trace_##call)__bpf_trace_##template,	\
 	.num_args	= COUNT_ARGS(args),				\
 	.writable_size	= size,						\
 };