[BPF] Do not generate BTF sections unnecessarily

If There is no types/non-empty strings, do not generate
.BTF section. If there is no func_info/line_info, do
not generate .BTF.ext section.

Signed-off-by: Yonghong Song <yhs@fb.com>

Differential Revision: https://reviews.llvm.org/D58936

llvm-svn: 355360