scripts/bpf: teach bpf_helpers_doc.py to dump BPF helper definitions

[ Upstream commit 7a387bed47f7e80e257d966cd64a3e92a63e26a1 ]

Enhance scripts/bpf_helpers_doc.py to emit C header with BPF helper
definitions (to be included from libbpf's bpf_helpers.h).

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Stable-dep-of: e37243b65d52 ("bpf, scripts: Correct GPL license name")
Signed-off-by: Sasha Levin <sashal@kernel.org>