libbpf: add __arg_xxx macros for annotating global func args

Add a set of __arg_xxx macros which can be used to augment BPF global
subprogs/functions with extra information for use by BPF verifier.

Acked-by: Eduard Zingerman <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/lib/bpf/bpf_helpers.h

···
 	!!sym;											\
 })
 
+#define __arg_ctx __attribute__((btf_decl_tag("arg:ctx")))
+#define __arg_nonnull __attribute((btf_decl_tag("arg:nonnull")))
+
 #ifndef ___bpf_concat
 #define ___bpf_concat(a, b) a ## b
 #endif