[MIPS] Add RDHWR with the SEL field for MIPS R6.

[MIPS] Add RDHWR with the SEL field for MIPS R6.

In Release 6 of the MIPS architecture [1], instruction RDHWR supports
a 3rd operand to serve as the 3-bit select field for the hardware
register.

[1] "MIPS Architecture for Programmers Volume II-A: The MIPS32
    Instruction Set Manual", Imagination Technologies Ltd., Document
    Number: MD00086, Revision 6.06, December 15, 2016, Section 3.2
    "Alphabetical List of Instructions", pp. 332-334

opcodes/
	* mips-opc.c (mips_builtin_opcodes): Add RDHWR rt rd sel.

gas/
	* testsuite/gas/mips/mips.exp: Run hwr-names test.
	* testsuite/gas/mips/hwr-names.s: Add test cases for RDHWR with
	the SEL field.
	* testsuite/gas/mips/mipsr6@hwr-names.d: New file.