libcflat: clean up and complete long division routines

Avoid possible uninitialized variables on machines where
division by zero does not trap.  Add __divmoddi4, and
use it in __moddi3 and __divdi3.

Reported-by: Alexandru Elisei <alexandru.elisei@arm.com>
Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>