xtensa: optimize find_removed_literal

xtensa: optimize find_removed_literal

find_removed_literal uses linear search to find removed literal by its
VMA. The list of literals is fixed at that point, build an ordered index
array and use binary search instead.

Original profile:

% time    self  children    called     name
-----------------------------------------
         56.72    0.00  297578/669392      translate_reloc
         70.86    0.00  371814/669392      relax_section
  67.9  127.58    0.00  669392         find_removed_literal
-----------------------------------------

Same data, after optimization:

% time    self  children    called     name
-----------------------------------------
          0.00    0.00  297578/669392      translate_reloc
          0.00    0.00  371814/669392      relax_section
   0.0    0.00    0.00  669392         find_removed_literal
          0.00    0.00   23838/23838       map_removed_literal
-----------------------------------------

2015-04-03  Max Filippov  <jcmvbkbc@gmail.com>
bfd/
	* elf32-xtensa.c (removed_literal_map_entry): new typedef.
	(removed_literal_map_entry_struct): new structure.
	(removed_literal_list_struct): add new fields: n_map and map.
	(map_removed_literal, removed_literal_compare): new functions.
	(find_removed_literal): build index array for literals ordered
	by VMA, use binary search to find removed literal.