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

This patch sync the uapi bpf.h and btf.h to tools.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>