core: Implement the maddhd, maddhdu and maddld instructions

core: Implement the maddhd, maddhdu and maddld instructions

These instructions use major opcode 4 and have a third GPR input
operand, so we need a decode table for major opcode 4 and some
plumbing to get the RC register operand read.

The multiply-add instructions use the same insn_type_t values as the
regular multiply instructions, and we distinguish in execute1 by
looking at the major opcode.  This turns out to be convenient because
we don't have to add any cases in the code that handles the output of
the multiplier, and it frees up some insn_type_t values.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>