MIPS: Add microMIPS XPA support

MIPS: Add microMIPS XPA support

Add support for the base and Virtualization ASE microMIPS instructions
as per the architecture specifications[1][2][3][4].

Most of this change by Andrew Bennett.

[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", p. 340

[2] "microMIPS32 Architecture for Programmers Volume IV-i:
    Virtualization Module of the microMIPS32 Architecture", MIPS
    Technologies, Inc., Document Number: MD00848, Revision 1.06,
    December 10, 2013, Section 6.1 "Overview", pp. 133, 136

[3] "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. 415, 444

[4] "microMIPS64 Architecture for Programmers Volume IV-i:
    Virtualization Module of the microMIPS64 Architecture", MIPS
    Technologies, Inc., Document Number: MD00849, Revision 1.06,
    December 10, 2013, Section 6.1 "Overview", pp. 134-135, 139-140

	binutils/
	* NEWS: Mention microMIPS XPA support.

	opcodes/
	* micromips-opc.c (XPA, XPAVZ): New macros.
	(micromips_opcodes): Add "mfhc0", "mfhgc0", "mthc0" and
	"mthgc0".

	gas/
	* config/tc-mips.c (mips_ases): Add microMIPS XPA support.
	* testsuite/gas/mips/micromips@xpa.d: New test.
	* testsuite/gas/mips/mips.exp: Run the new test.  Enable
	`xpa-virt-err' test for `micromips'.