rs6000: Add %wD constraint and predicate for accumulators

The future processor may introduce new set of registers for
accumulators. This patch adds a constraint and predicate for the
accumulator registers which can be used by the dense math and mma
patterns.

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

gcc/ChangeLog:
	* config/rs6000/constraints.md
	(rs6000_constraints[RS6000_CONSTRAINT_wD]): New wD constraint.
	* config/rs6000/predicates.md (accumulator_operand): New predicate.
	* config/rs6000/rs6000.cc (rs6000_debug_reg_global): Support wD
	register class.
	(rs6000_init_hard_regno_mode_ok): Map wD to FLOAT_REGS.
	* config/rs6000/rs6000.h (enum r6000_reg_class_enum): Add wD constraint.
	* doc/md.texi: Document the new constraint.