Add macro expansions for ADD, SUB, DADD and DSUB for MIPS r6

Add macro expansions for ADD, SUB, DADD and DSUB for MIPS r6

Release 6 of the MIPS architecture does not have an ADDI instruction.
ADD/SUB instructions with immediate operands can be expanded to load
and immediate value and then perform the operation.

gas/
	* config/tc-mips.c (macro) <M_ADD_I, M_SUB_I, M_DADD_I, M_DSUB_I>:
	Add expansions for MIPS r6.
	* testsuite/gas/mips/add.s: Enable tests for R6.
	* testsuite/gas/mips/daddi.s: Annotate to test DADD for R6.
	* testsuite/gas/mips/mipsr6@add.d: Likewise.
	* gas/testsuite/gas/mips/mipsr6@dadd.d: New test.
	* gas/testsuite/gas/mips/mips.exp: Run the new test.

opcodes/
        * mips-opc.c (mips_opcodes): Enable ADD, SUB, DADD and DSUB
	macros for R6.