config: Add a check for fastmem only used with Atomic CPU

This patch adds an additional check to ensure that the fastmem option
is only used if the system is using the Atomic CPU.

configs/example/fs.py

@@ -128,8 +128,11 @@ else:
     test_sys.iobridge.master = test_sys.membus.slave
 
 # Sanity check
-if options.fastmem and (options.caches or options.l2cache):
-    fatal("You cannot use fastmem in combination with caches!")
+if options.fastmem:
+    if TestCPUClass != AtomicSimpleCPU:
+        fatal("Fastmem can only be used with atomic CPU!")
+    if (options.caches or options.l2cache):
+        fatal("You cannot use fastmem in combination with caches!")
 
 for i in xrange(np):
     if options.fastmem: