Patch to enable build on s390x

[Torbjorn Granlund]

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

  Yes that's intentional. clock_gettime should be the better choice for both platforms.
  Using STCK directly has a higher resolution but is not stopped when the CPU is taken away
  from your system so in general the results will not be reliable when trying to measure the
  CPU cycles spent for a certain workload.
  
This is the case for many accurate measurement mechanisms under the
kernel Linux.  E.g., the x86 rdtsc is not managed properly by that
kernel.

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.

-- 
Torbjörn