bpf, sk_msg: Add some generic helpers that may be useful from sk_msg

bpf, sk_msg: Add some generic helpers that may be useful from sk_msg

Add these generic helpers that may be useful to use from sk_msg programs.
The helpers do not depend on ctx so we can simply add them here,

 BPF_FUNC_perf_event_output
 BPF_FUNC_get_current_uid_gid
 BPF_FUNC_get_current_pid_tgid
 BPF_FUNC_get_current_cgroup_id
 BPF_FUNC_get_current_ancestor_cgroup_id
 BPF_FUNC_get_cgroup_classid

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/159033903373.12355.15489763099696629346.stgit@john-Precision-5820-Tower
Signed-off-by: Alexei Starovoitov <ast@kernel.org>