build.sh: allow arch -x86_64 builds on Apple Silicon.
build.sh: allow arch -x86_64 builds on Apple Silicon.

mrjbq7/factor,bpollack/factor,bpollack/factor,bpollack/factor,tgunr/factor,bpollack/factor,tgunr/factor,factor/factor,factor/factor,mrjbq7/factor,bpollack/factor,factor/factor,tgunr/factor,factor/factor,tgunr/factor,mrjbq7/factor,mrjbq7/factor,mrjbq7/factor,factor/factor,bpollack/factor,mrjbq7/factor,tgunr/factor,bpollack/factor,factor/factor,tgunr/factor