bpf: btf: Sync uapi btf.h to tools

This patch sync the uapi btf.h to tools/

Fixes: 36fc3c8c bpf: btf: Clean up BTF_INT_BITS() in uapi btf.h
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>