MIPS16: Add ASMACRO instruction support

MIPS16: Add ASMACRO instruction support

Add ASMACRO instruction support as per the MIPS16e ASE architecture
specifications [1][2], completing MIPS16e instruction set support.

[1] "MIPS32 Architecture for Programmers, Volume IV-a: The MIPS16e
    Application-Specific Extension to the MIPS32 Architecture", MIPS
    Technologies, Inc., Document Number: MD00076, Revision 2.63, July
    16, 2013, Section 4.1 "MIPS16e Instruction Descriptions", p. 65

[2] "MIPS64 Architecture for Programmers, Volume IV-a: The MIPS16e
    Application-Specific Extension to the MIPS64 Architecture", MIPS
    Technologies, Inc., Document Number: MD00077, Revision 2.60, June
    25, 2008, Section 1.1 "MIPS16e Instruction Descriptions", p. 66

	include/
	* opcode/mips.h: Document `0', `1', `2', `3', `4' and `s'
	operand codes.

	opcodes/
	* mips16-opc.c (decode_mips16_operand): Add `0', `1', `2', `3',
	`4' and `s' operand codes.
	(mips16_opcodes): Add "asmacro" entry.

	binutils/
	* testsuite/binutils-all/mips/mips16-extend-insn.d: Update for
	ASMACRO support.

	gas/
	* testsuite/gas/mips/mips16-asmacro.d: New test.
	* testsuite/gas/mips/mips16-32@mips16-asmacro.d: New test.
	* testsuite/gas/mips/mips16-64@mips16-asmacro.d: New test.
	* testsuite/gas/mips/mips16-asmacro.s: New test source.
	* testsuite/gas/mips/mips.exp: Run the new tests.