config: Add a --without-python option to build process

Add the ability to build libgem5 without embedded Python or the
ability to configure with Python.

This is a prelude to a patch to allow config.ini files to be loaded
into libgem5 using only C++ which would make embedding gem5 within
other simulation systems easier.

This adds a few registration interfaces to things which cross
between Python and C++.  Namely: stats dumping and SimObject resolving

src/python/m5/stats/__init__.py

@@ -41,6 +41,7 @@ def initText(filename, desc=True):
 
 def initSimStats():
     internal.stats.initSimStats()
+    internal.stats.registerPythonStatsHandlers()
 
 names = []
 stats_dict = {}