selftests/bpf: add __arg_ctx BTF rewrite test

Add a test validating that libbpf uploads BTF and func_info with
rewritten type information for arguments of global subprogs that are
marked with __arg_ctx tag.

Suggested-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240104013847.3875810-10-andrii@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>