[BPF] handle external global properly

Previous commit 6bc58e6d3dbd ("[BPF] do not generate unused local/global types")
tried to exclude global variable from type generation. The condition is:
     if (Global.hasExternalLinkage())
       continue;
This is not right. It also excluded initialized globals.

The correct condition (from AssemblyWriter::printGlobal()) is:
  if (!GV->hasInitializer() && GV->hasExternalLinkage())
    Out << "external ";

Let us do the same in BTF type generation. Also added a test for it.

Signed-off-by: Yonghong Song <yhs@fb.com>
llvm-svn: 356279