bpf: btf: Add BTF_KIND_FUNC and BTF_KIND_FUNC_PROTO

This patch adds BTF_KIND_FUNC and BTF_KIND_FUNC_PROTO
to support the function debug info.

BTF_KIND_FUNC_PROTO must not have a name (i.e. !t->name_off)
and it is followed by >= 0 'struct bpf_param' objects to
describe the function arguments.

The BTF_KIND_FUNC must have a valid name and it must
refer back to a BTF_KIND_FUNC_PROTO.

The above is the conclusion after the discussion between
Edward Cree, Alexei, Daniel, Yonghong and Martin.

By combining BTF_KIND_FUNC and BTF_LIND_FUNC_PROTO,
a complete function signature can be obtained.  It will be
used in the later patches to learn the function signature of
a running bpf program.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>