[patch][gold] Remove empty Merge_map class

[Rafael Espíndola]

When looking at the constant merging code to see if the
mapping could be reused for gc, the most confusing part for
me was the empty Merge_map class.

It is only used as a key to implement is_merge_section_for (which is
only used by find_starting_output_address).

Since it is just a key, we can use the Output_section_data as a key
instead. This patch does that. IMHO removing the indirection makes
things a bit easier to understand. It also removes the need for the
virtual do_is_merge_section_for.

Last but not least, it makes some inefficiencies easier to spot. For
example,  Output_merge_data::do_add_input_section handles only one
input section and therefore only one merge map, but it does

 object->add_merge_mapping(this, shndx, i, entsize, k);

in a loop. It could extract Input_merge_map* before the loop.

Cheers,
Rafael
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