common: factor -j --nproc to all builds

build-m5

@@ -1,10 +1,6 @@
 #!/usr/bin/env python3
 
-import multiprocessing
 import os
-import platform
-import subprocess
-import time
 
 import common
 
@@ -19,7 +15,7 @@ class M5Component(common.Component):
             arch = args.arch
         return [
             'make',
-            '-j', str(multiprocessing.cpu_count()),
+            '-j', str(args.nproc),
             '-f', 'Makefile.{}'.format(arch),
             'CC={}'.format(cc),
             'LD={}'.format(ld),