s390x/tcg: Fix parts of IEEE exception handling

s390x/tcg: Fix parts of IEEE exception handling

Many things are wrong and some parts cannot be fixed yet. Fix what we
can fix easily and add two FIXMEs:

The fpc flags are not updated in case an exception is actually injected.
Inexact exceptions have to be handled separately, as they are the only
exceptions that can coexist with underflows and overflows.

I reread the horribly complicated chapters in the PoP at least 5 times
and hope I got it right.

For references:
- z14 PoP, 9-18, "IEEE Exceptions"
- z14 PoP, 19-9, Figure 19-8

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20190218122710.23639-5-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>