[MIPS] Add load-link, store-conditional paired instructions

[MIPS] Add load-link, store-conditional paired instructions

Add several baseline MIPS32R6[1] and MIPS64R6[2] instructions
that were omitted from the initial spec.  These instructions
are optional in implementations but not associated with any
ASE or pseudo-ASE.  Their presence is indicated by the XNP bit
in the Config5 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. 228-229, pp. 354-357.

[2] "MIPS Architecture for Programmers Volume II-A: The MIPS64
     Instruction Set Manual", Imagination Technologies Ltd., Document
     Number: MD00087, Revision 6.06, December 15, 2016, Section 3.2
     "Alphabetical List of Instructions", pp. 289-290 and pp. 458-460.

gas/
	* config/tc-mips.c (macro) <M_LLWP_AB, M_LLDP_AB, M_SCWP_AB,
	M_SCDP_AB>: New cases and expansions for paired instructions.
	* testsuite/gas/mips/llpscp-32.s: New test source.
	* testsuite/gas/mips/llpscp-64.s: Likewise.
	* testsuite/gas/mips/llpscp-32.d: New test.
	* testsuite/gas/mips/llpscp-64.d: Likewise.
	* testsuite/gas/mips/mips.exp: Run the new tests.
	* testsuite/gas/mips/r6.s: Add new instructions to test source.
	* testsuite/gas/mips/r6-64.s: Likewise.
	* testsuite/gas/mips/r6-64-n32.d: Check new instructions.
	* testsuite/gas/mips/r6-64-n64.d: Likewise.
	* testsuite/gas/mips/r6-n32.d: Likewise.
	* testsuite/gas/mips/r6-n64.d: Likwwise.
	* testsuite/gas/mips/r6.d: Likewise.

include/
	* opcode/mips.h (M_LLWP_AB, M_LLDP_AB): New enum values.
	(M_SCWP_AB, M_SCDP_AB): Likewise.

opcodes/
	* mips-opc.c (mips_builtin_opcodes): Add llwp, lldp, scwp, scdp.