[PATCH] benchtests: Run _Float128 tests only on architectures that support it

[Paul Zimmermann]

       Dear Arjun,

some comments on your patch:

* the changes to benchtests/Makefile and benchtests/*f128-inputs look ok to me
* for configure.ac I'm not fluent enough in the autotools
* I wonder why you also patch configure, isn't this file automatically
  generated?
* finally "find . -type f -exec grep \-l __float128 {} \;" finds several other
  instances of __float128 (sysdeps/x86_64/fpu/math-tests-snan.h for example).
  Should some of those be changed to _Float128 too?

Best regards,
Paul