bpftool: Add support for BTF_KIND_TAG

Added bpftool support to dump BTF_KIND_TAG information.
The new bpftool will be used in later patches to dump
btf in the test bpf program object file.

Currently, the tags are not emitted with
  bpftool btf dump file <path> format c
and they are silently ignored.  The tag information is
mostly used in the kernel for verification purpose and the kernel
uses its own btf to check. With adding these tags
to vmlinux.h, tags will be encoded in program's btf but
they will not be used by the kernel, at least for now.
So let us delay adding these tags to format C header files
until there is a real need.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210914223031.246951-1-yhs@fb.com