Do not abuse the global ARCH variable as a local temporary
Do not abuse the global ARCH variable as a local temporary

Setting it with a simple "uname -m" just to be able to decide whether to compile getarch.c with -march=native
may actually keep getarch from doing a proper probe. Fixes #2231, a regression caused by #2110
xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS