pause techniques on many architectures

#if defined(__SSE2__)  // AMD and Intel
  #include <emmintrin.h>
  _mm_pause();
#elif defined(__i386__) || defined(__x86_64__)
  asm volatile("pause");
#elif defined(__aarch64__)
  asm volatile("wfe");
#elif defined(__armel__) || defined(__ARMEL__) 
  asm volatile ("nop" ::: "memory");  // default operation - does nothing => Might lead to passive spinning.
#elif defined(__arm__) || defined(__aarch64__) // arm big endian / arm64
  __asm__ __volatile__ ("yield" ::: "memory");
#elif defined(__ia64__)  // IA64
  __asm__ __volatile__ ("hint @pause");
#elif defined(__powerpc__) || defined(__ppc__) || defined(__PPC__) // PowerPC
   __asm__ __volatile__ ("or 27,27,27" ::: "memory");
#else  // everything else.
   asm volatile ("nop" ::: "memory");  // default operation - does nothing => Might lead to passive spinning.
#endif