tcg/s390x: Add host vector framework

Add registers and function stubs.  The functionality
is disabled via squashing s390_facilities[2] to 0.

We must still include results for the mandatory opcodes in
tcg_target_op_def, as all opcodes are checked during tcg init.

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