s390x/tcg: Handle SET FPC AND LOAD FPC 3-bit BFP rounding modes

s390x/tcg: Handle SET FPC AND LOAD FPC 3-bit BFP rounding modes

We already forward the 3 bits correctly in the translation functions. We
also have to handle them properly and check for specification
exceptions.

Setting an invalid rounding mode (BFP only, all DFP rounding modes)
results in a specification exception. Setting unassigned bits in the
fpc, results in a specification exception.

This fixes LOAD FPC (AND SIGNAL), SET FPC (AND SIGNAL). Also for,
SET BFP ROUNDING MODE, 3-bit rounding mode is now explicitly checked.

Note: TCG_CALL_NO_WG is required for sfpc handler, as we now inject
exceptions.

We won't be modeling abscence of the "floating-point extension facility"
for now, not necessary as most take the facility for granted without
checking.

z14 PoP, 9-23, "LOAD FPC"
    When the floating-point extension facility is
    installed, bits 29-31 of the second operand must
    specify a valid BFP rounding mode and bits 6-7,
    14-15, 24, and 28 must be zero; otherwise, a
    specification exception is recognized.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20190218122710.23639-9-david@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>