Add VSX Scalar Move Instructions

This patch adds the VSX scalar move instructions:

  - xsabsdp (Scalar Absolute Value Double-Precision)
  - xsnabspd (Scalar Negative Absolute Value Double-Precision)
  - xsnegdp (Scalar Negate Double-Precision)
  - xscpsgndp (Scalar Copy Sign Double-Precision)

A common generator macro (VSX_SCALAR_MOVE) is added since these
instructions vary only slightly from each other.

Macros to support VSX XX2 and XX3 form opcodes are also added.
These macros handle the overloading of "opcode 2" space (instruction
bits 26:30) caused by AX and BX bits (29 and 30, respectively).

V3: Per feedback from Paolo Bonzini, moved the sign mask into a
temporary and used andc.

Signed-off-by: Tom Musta <tommusta@gmail.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
