This is similiar to what has been done in [af2ba9c8/integer-gmp] for
`gmpz_tdiv_{q,r,qr}_ui` (re #8647); However, the gain is more modest
here, as performance-conscious code tends to use `quot`/`rem` rather
than `div`/`mod`:

     Program    Size    Allocs   Runtime   Elapsed  TotalMem
 -------------------------------------------------------------
   primetest   +0.3%     -2.4%      0.06      0.06     +0.0%
         rsa   +0.2%     -3.3%      0.02      0.02     +0.0%

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>