Hi Cary,

Now that Input_merge_map has an Output_section_data, we can use it in
implementing find_merge_section and replace the only use of
is_merge_section_for with it.

This simplifies the code a bit and is faster. Linking chromium goes from

  6173.193320      task-clock (msec)         #    0.999 CPUs utilized
          ( +-  0.62% )

to

 5693.481663      task-clock (msec)         #    0.999 CPUs utilized
         ( +-  0.49% )

Cheers,
Rafael