[ARC] Check if an input asm file is rf16 compliant

Check if an input asm file is rf16 compliant; if not, and the tag says
otherwise, fix the tag and emit a warning.

gas/
2017-09-20  Claudiu Zissulescu <claziss@synopsys.com>

	* config/tc-arc.c (rf16_only): New static variable.
	(autodetect_attributes): Check if we are rf16 compliant.
	(arc_set_public_attributes): Fix and emit the warning is required.
	* testsuite/gas/arc/attr-rf16.d: New file.
	* testsuite/gas/arc/attr-rf16.err: Likewise.
	* testsuite/gas/arc/attr-rf16.s: Likewise.