Add __s390x__ compiler define for s390 builds. (#26233)
Summary:
pytorch builds fail on 390 architecture because
in simd.h the ifdef macros default to an x86 asm instruction.
This patchs adds an ifdef __s390x__ to be able to build on s390.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26233
Differential Revision: D17392714
Pulled By: soumith
fbshipit-source-id: 037672bfea64fc5e52da2390d93b973534137c12
