common: factor -j --nproc to all builds

build-userland

@@ -1,6 +1,5 @@
 #!/usr/bin/env python3
 
-import multiprocessing
 import os
 import platform
 import shutil
@@ -42,7 +41,7 @@ has the OpenBLAS libraries and headers installed.
             (
                 [
                     'make',
-                    '-j', str(multiprocessing.cpu_count()),
+                    '-j', str(args.nproc),
                     'CC={}'.format(cc),
                     'CXX={}'.format(cxx),
                     'PKG_CONFIG={}'.format(common.buildroot_pkg_config),