Make allocations report usable size.

Work-in-progress to allow arrays to fill usable size. Bug: 13028925.
Use C++11's override keyword on GCC >= 2.7 to ensure that we override GC and
allocator methods.
Move initial mirror::Class set up into a Functor so that all allocated objects
have non-zero sizes. Use this property to assert that all objects are never
larger than their usable size.
Other bits of GC related clean-up, missing initialization, missing use of
const, hot methods in .cc files, "unimplemented" functions that fail at
runtime in header files, reducing header file includes, move valgrind's space
into its own files, reduce number of array allocation routines.

Change-Id: Id5760041a2d7f94dcaf17ec760f6095ec75dadaa
diff --git a/runtime/gc/collector/semi_space.h b/runtime/gc/collector/semi_space.h
index ba97376..c164c5f 100644
--- a/runtime/gc/collector/semi_space.h
+++ b/runtime/gc/collector/semi_space.h
@@ -275,7 +275,7 @@
   // When true, the generational mode (promotion and the bump pointer
   // space only collection) is enabled. TODO: move these to a new file
   // as a new garbage collector?
-  bool generational_;
+  const bool generational_;
 
   // Used for the generational mode. the end/top of the bump
   // pointer space at the end of the last collection.