MIPS: Add microMIPS R5 support

MIPS: Add microMIPS R5 support

Add base microMIPS Release 5 ISA support and the ERETNC instruction in
particular, as per the architecture specifications[1][2].

Most of this change by Andrew Bennett.

References:

[1] "MIPS Architecture for Programmers Volume II-B: The microMIPS32
    Instruction Set", MIPS Technologies, Inc., Document Number: MD00582,
    Revision 5.04, January 15, 2014, Section 5.5 "Recoded 32-Bit
    Instructions", pp. 266-267

[2] "MIPS Architecture for Programmers Volume II-B: The microMIPS64
    Instruction Set", MIPS Technologies, Inc., Document Number: MD00594,
    Revision 5.04, January 15, 2014, Section 5.5 "Recoded 32-Bit
    Instructions", pp. 326-327

	binutils/
	* NEWS: Mention microMIPS Release 5 ISA support.

	opcodes/
	* micromips-opc.c (I36): New macro.
	(micromips_opcodes): Add "eretnc".

	gas/
	* testsuite/gas/mips/micromips@r5.d: New test.
	* testsuite/gas/mips/mips.exp: Run the new test.