Patch to enable build on s390x

[Torbjorn Granlund]

Andreas Krebbel <krebbel at linux.vnet.ibm.com> writes:

  > Our experience is that it is nevertheless better to use rdtsc than
  > low-accuracy measurement mechanisms.  Most GMP threshold parameters are
  > for computations that take just a few nanoseconds.
  
  From the comments in time.c I expected that this is compensated by running the tests more
  often and divide the time by the number of runs.
  
  From a first glance using stck seems to require the CPU frequency and this is not that
  easy to determine. The only way I can think of is putting a cpu table into the code which
  would need updating whenever a new CPU appears. So for now clock_gettime appears to be a
  bit easier to me. Btw. libatlas also seems to rely on clock_gettime - however their tests
  seem to run much longer.
  
OK, so let's go with clock_gettime for this system.

-- 
Torbjörn