message: add support for riscv
diff: namespace bee { inline void cpu_relax() { asm volatile("yield" ::: "memory"); }}
  #elif defined(__arm__)
  namespace bee { inline void cpu_relax() { asm volatile("":::"memory"); }}
  +#elif defined(__riscv)
  + namespace bee { inline void cpu_relax() {
  + int dummy;
  + asm volatile ("div %0, %0, zero" : "=r" (dummy));
  + asm volatile ("" ::: "memory");
  + }}
  #else
  #error unsupport platform
  #endif
  