[ARC] Dynamic relocs verification for dynindx == -1.

[ARC] Dynamic relocs verification for dynindx == -1.

bfd/
2016-04-05  Cupertino Miranda  <cmiranda@synopsys.com>

	* elf32-arc.c (name_for_global_symbol): Added assert to check for
	symbol index.
	(elf_arc_relocate_section): Added and changed asserts, validating
	the synamic symbol index.
	(elf_arc_finish_dynamic_symbol): Do not fill the dynamic
	relocation if symbol has dynindx set to -1.