s390x/tcg: Fault-safe memset

Replace fast_memset() by access_memset(), that first tries to probe
access to all affected pages (maximum is two). We'll use the same
mechanism for other types of accesses soon.

Only in very rare cases (especially TLB_NOTDIRTY), we'll have to
fallback to ld/st helpers. Try to speed up that case as suggested by
Richard.

We'll rework most involved handlers soon to do all accesses via new
fault-safe helpers, especially MVC.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>