Conditional configuration sections: expose new 'multiarch' value in sys

To be dropped once all buildout in the wild are able to upgrade to a
version that supports new names in expression of conditional sections
(see previous commit).

src/zc/buildout/buildout.py

@@ -1872,6 +1872,7 @@ class _default_globals(dict):
             context = self.context
         except AttributeError:
             context = self.context = cls._default_globals()
+            context['sys'] = _sysproxy(self)
         try:
             return context[key]
         except KeyError as e:
@@ -1891,6 +1892,18 @@ class _default_globals(dict):
         del self['__doing__']
         return m
 
+class _sysproxy(object): # BBB: alternate/temporary way to get multiarch value
+
+    def __init__(self, default_globals):
+        self.__default_globals = default_globals
+
+    def __getattr__(self, name):
+        if name == '_multiarch':
+            default_globals = self.__default_globals
+            setattr(sys, name, getattr(default_globals, name[1:])())
+            default_globals.context['sys'] = sys
+        return getattr(sys, name)
+
 
 def _open(base, filename, dl_options, override, seen=None, processing=None):
     """Open a configuration file and return the result as a dictionary,