Use usleep instead of sched_yield by default
Use usleep instead of sched_yield by default

sched_yield only burns cpu cycles, fixes #900,  see also #923, #1560
xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS