[ARC] Enhance enter/leave mnemonics.

[ARC] Enhance enter/leave mnemonics.

enter/leave mnemonics are enhanced to not only accept register ranges
but also single register (i.e., r13) or even no GPR register at all.

gas/
2017-04-25  Claudiu Zissulescu  <claziss@synopsys.com>

	* testsuite/gas/arc/leave_enter.d: Update test.
	* testsuite/gas/arc/leave_enter.s: Likewise.

opcodes/
2017-04-25  Claudiu Zissulescu  <claziss@synopsys.com>

	* arc-dis.c (print_insn_arc): Smartly print enter/leave mnemonics.
	* arc-opc.c (insert_r13el): New function.
	(R13_EL): Define.
	* arc-tbl.h: Add new enter/leave variants.