python,test: Fixed boot test in python3 by removing map

In Python3 `map(lambda c: c.createThreads(), self.cpu)` does not
execute `c.createThreads()`. This has been replaced with a for-loop
which does work. Without this fix, the boot tests do not run in python3.

Change-Id: I50d6c85ec4435ee04e248ea8bc4a3b4cc17c88fa
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29456
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>

tests/gem5/x86-boot-tests/system/system.py

@@ -118,7 +118,8 @@ class MySystem(System):
         else:
             m5.fatal("No CPU type {}".format(cpu_type))
 
-        map(lambda c: c.createThreads(), self.cpu)
+        for c in self.cpu:
+            c.createThreads()
 
     def setDiskImages(self, img_path_1, img_path_2):
         disk0 = CowDisk(img_path_1)