Use nproc and sysctl -n hw.cpu for default initializing CORES

This avoids having to pass --build-arg CORES=n when building the
docker images.

build-llvm.sh

@@ -22,6 +22,8 @@ if [ -z "$PREFIX" ]; then
     exit 1
 fi
 
+: ${CORES:=$(nproc 2>/dev/null)}
+: ${CORES:=$(sysctl -n hw.ncpu 2>/dev/null)}
 : ${CORES:=4}
 
 if [ ! -d llvm ]; then