For BC labels, emit empty data section instead of empty proc.

For BC labels, emit empty data section instead of empty proc.

This fixes two bugs:

    - The new code generator doesn't like procedures with
      empty graphs, and panicked in labelAGraph.

    - LLVM optimizes away empty procedures but not empty
      data sections, so now the backwards-compatibility
      labels actually work with -fllvm.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>