Attempt to recover gdb bot after r174445.

Attempt to recover gdb bot after r174445.

Failure: undefined symbol 'Lline_table_start0'.
Root-cause: we use a symbol subtraction to calculate at_stmt_list, but
the line table entries are not dumped in the assembly.
Fix: use zero instead of a symbol subtraction for Compile Unit 0.

llvm-svn: 174479