s390x/tcg: Prepare for IEEE-inexact-exception control (XxC)

Some instructions allow to suppress IEEE inexact exceptions.

z14 PoP, 9-23, "Suppression of Certain IEEE Exceptions"
    IEEE-inexact-exception control (XxC): Bit 1 of
    the M4 field is the XxC bit. If XxC is zero, recogni-
    tion of IEEE-inexact exception is not suppressed;
    if XxC is one, recognition of IEEE-inexact excep-
    tion is suppressed.

Especially, handling for overflow/unerflow remains as is, inexact is
reported along

z14 PoP, 9-23, "Suppression of Certain IEEE Exceptions"
    For example, the IEEE-inexact-exception control (XxC)
    has no effect on the DXC; that is, the DXC for IEEE-
    overflow or IEEE-underflow exceptions along with the
    detail for exact, inexact and truncated, or inexact and
    incremented, is reported according to the actual con-
    dition.

Follow up patches will wire it correctly up for the applicable
instructions.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20190218122710.23639-12-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>