POWER10: Fix multithreading check when USE_THREAD=0
POWER10: Fix multithreading check when USE_THREAD=0

This patch fixes an issue when OpenBLAS is compiled for TARGET=POWER10
and the flag USE_THREAD is set to 0.

The function `num_cpu_avail` is only available when USE_THREAD=1,
so SMP is defined.

xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS