target/s390x: add BAL and BALR instructions

target/s390x: add BAL and BALR instructions

These instructions are provided for compatibility purposes and are
used only by old software, in the new code BAS and BASR are preferred.
The difference between the old and new instruction exists only in the
24-bit mode.

In addition, fix BAS polluting high 32 bits of the first operand in
24- and 31-bit addressing modes.

Signed-off-by: Pavel Zbitskiy <pavel.zbitskiy@gmail.com>
Message-Id: <20180821025104.19604-3-pavel.zbitskiy@gmail.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>