bpf: sync {btf, bpf}.h uapi header from tools infrastructure

Pull in latest changes from both headers, so we can make use of
them in libbpf.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>