Darwin and fat builds

[Torbjorn Granlund]

  In the meantime, I have added testing of fat binaries on shell, reported
  under the alias tjockshell.  (But if course this is for the 5.0 repo.)
  I will enable it on some other machines too, so that all fat code paths
  are tested.

Now, many mode x86 machines do fat builds, pl see results at
http://gmplib.org/devel/testmachines.shtml.  (The build system tests the
gmp 5 repo in this period, remember.)

It turns out fat builds on Darwin wasn't really supported, neither in
32-bit or 64-bit mode, nether by the main gmp repo nor by the gmp 5
repo..

I have now pushed changes to make 64-bit mode work for gmp 5.  With your
change, it will probably work also for the main gmp repo.

I do not intend to work on making fat builds on 32-bit Darwin function.
It will require some messing about with relocations for several of the
files in mpn/x86.  At some point, this out to be fixed.

-- 
Torbjörn