target-ppc: Store Quadword

This patch adds support for the Store Quadword instruction in user mode.  Prior
to Power ISA 2.07, stq was legal only in privileged mode.  Support for Little
Endian mode is also new in ISA 2.07.

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