Optimize the performance of the group_setup function.

Optimize the performance of the group_setup function.

When processing a section that is a member of a group, the group
that contains it is looked up using a linear search. The resulting
O(n^2) complexity causes significant performance issues when
dealing with object files with very many groups.

By remembering the index of the last found group and restarting
the next search from that index, the search instead becomes O(n)
in common cases.

	* elf.c (setup_group): Optimize search for group by remembering
	last found group and restarting search at that index.
	* elf-bfd.h (struct elf_obj_tdata): Add group_search_offset field.