rs6000: Update existing mma insns to use wD constraint and update %A printer

This patch replaces the 'd' constraint that was used for accumulators
previously since they overlapped with the FPRs, to use 'wD' constraint
and the new accumulator_operand in the existing mma define_insns. The
behaviour without dense-math option will be same as before, only if
dense-math option is used will the DMR registers be used.

It also updates the %A operand printer to print the DMR register numbers
instead of FPR if dense-math option is enabled

2026-07-16  Avinash Jayakar  <avinashd@linux.ibm.com>
	    Vijay Shankar  <vijay@linux.ibm.com>

gcc/ChangeLog:
	* config/rs6000/mma.md (mma_xxsetaccz): Use wD constraint and
	accumulator operand.
	(mma_<vv>): Likewise.
	(mma_<avv>): Likewise.
	(mma_<pv>): Likewise.
	(mma_<apv>): Likewise.
	(mma_<vvi4i4i8>): Likewise.
	(mma_<avvi4i4i8>): Likewise.
	(mma_<vvi4i4i2>): Likewise.
	(mma_<avvi4i4i2>): Likewise.
	(mma_<vvi4i4>): Likewise.
	(mma_<avvi4i4>): Likewise.
	(mma_<pvi4i2>): Likewise.
	(mma_<apvi4i2>): Likewise.
	(mma_<vvi4i4i4>): Likewise.
	(mma_<avvi4i4i4>): Likewise.
	* config/rs6000/rs6000.cc (print_operand): Print DMR register
	numbers if dense math option enabled.