unbalanced multiplication and division

[Paul Zimmermann]

> You should know that our focus have become 64-bit machines since GMP
> 4.3.  That does not mean we do not optimise  for 32-bit machines,
> only that they might get less attention.

on http://www.loria.fr/~zimmerma/{plot1000.ps,plot10000.ps} you will
find similar curves obtained on a 64-bit Core 2, after "make tune":

* plot1000.ps shows a similar behaviour (size of the product or dividend
  is 1000 limbs)

* plot10000.ps shows a similar behaviour for the division, and a decrease
  at about 1100 limbs for the multiplication, and then the time is constant,
  probably due to the use of an FFT of total size 10000+ limbs -> the tuning
  should be improved here.

Paul