Fix mac build nproc command not found (#18362)

* change nproc 8

paddle/scripts/paddle_build.sh

@@ -292,13 +292,17 @@ function check_style() {
 #=================================================
 
 function build_base() {
+    if [ "$SYSTEM" == "Linux" ];then
       parallel_number=`nproc`
-    if [[ "$1" != "" ]]; then
+    else
+      parallel_number=8
+    fi
+    if [ "$1" != "" ]; then
       parallel_number=$1
     fi
     make clean
     make -j ${parallel_number}
-    make install -j `nproc`
+    make install -j ${parallel_number}
 }