code>, <code>mpn_udiv_qrnnd</code>: Return a
     structure like <code>div_t</code> to avoid going through memory, in
     particular helping RISCs that don't do store-to-load forwarding.  Clearly
     this is only possible if the ABI returns a structure of two
     <code>mp_limb_t</code>s in registers.
     <br>
     On PowerPC, structures are returned in memory on AIX and Darwin.  In SVR4
     they're returned in registers, except that draft SVR4 had said memory, so
     it'd be prudent to check which is 