bpf: btf: Sync bpf.h and btf.h to tools/

This patch sync up the bpf.h and btf.h to tools/

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