[ARC] Update merging attributes.

Some attributes were ignored during merging. Fix that, and add some
errors.

bfd/
2017-09-19  Claudiu Zissulescu <claziss@synopsys.com>

	* elf32-arc.c (arc_elf_merge_attributes): Fix merge attributes.

ld/
2017-09-19  Claudiu Zissulescu <claziss@synopsys.com>

	* testsuite/ld-arc/attr-merge-6.d: New file.
	* testsuite/ld-arc/attr-merge-6a.s: Likewise.
	* testsuite/ld-arc/attr-merge-6b.s: Likewise.
	* testsuite/ld-arc/attr-merge-6bis.d: Likewise.