target/s390x: implement mvcos instruction

This adds support for the MOVE WITH OPTIONAL SPECIFICATIONS (MVCOS)
instruction. Allow to enable it for the qemu cpu model using

qemu-system-s390x ... -cpu qemu,mvcos=on ...

This allows to boot linux kernel that uses it for uacccess.

We are missing (as for most other part) low address protection checks,
PSW key / storage key checks and support for AR-mode.

We fake an ADDRESSING exception when called from problem state (which
seems to rely on PSW key checks to be in place) and if AR-mode is used.
user mode will always see a PRIVILEDGED exception.

This patch is based on an original patch by Miroslav Benes (thanks!).

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170614133819.18480-3-david@redhat.com>
Signed-off-by: Richard Henderson <rth@twiddle.net>
