s390x: Use uint64_t for vector registers

s390x: Use uint64_t for vector registers

CPU_DoubleU is primarily used to reinterpret between integer and floats.
We don't really need this functionality. So let's just keep it simple
and use an uint64_t.

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