kbuild, btf: Fix dependencies for DEBUG_INFO_BTF

Currently turning on DEBUG_INFO_SPLIT when DEBUG_INFO_BTF is also
enabled will produce invalid btf file, since gen_btf function in
link-vmlinux.sh script doesn't handle *.dwo files.

Enabling DEBUG_INFO_REDUCED will also produce invalid btf file,
and using GCC_PLUGIN_RANDSTRUCT with BTF makes no sense.

Fixes: e83b9f55 ("kbuild: add ability to generate BTF type info for vmlinux")
Reported-by: Jann Horn <jannh@google.com>
Reported-by: Liu Yiding <liuyd.fnst@cn.fujitsu.com>
Signed-off-by: Slava Bacherikov <slava@bacher09.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: KP Singh <kpsingh@google.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200402204138.408021-1-slava@bacher09.org