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: NJann Horn <jannh@google.com>
Reported-by: NLiu Yiding <liuyd.fnst@cn.fujitsu.com>
Signed-off-by: NSlava Bacherikov <slava@bacher09.org>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Reviewed-by: NKees Cook <keescook@chromium.org>
Acked-by: NKP Singh <kpsingh@google.com>
Acked-by: NAndrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200402204138.408021-1-slava@bacher09.org

lib/Kconfig.debug

@@ -241,6 +241,8 @@ config DEBUG_INFO_DWARF4
 config DEBUG_INFO_BTF
 	bool "Generate BTF typeinfo"
 	depends on DEBUG_INFO
+	depends on !DEBUG_INFO_SPLIT && !DEBUG_INFO_REDUCED
+	depends on !GCC_PLUGIN_RANDSTRUCT || COMPILE_TEST
 	help
 	  Generate deduplicated BTF type information from DWARF debug info.
 	  Turning this on expects presence of pahole tool, which will convert