Improve the generational mode.

- Turn the compile-time flags for generational mode into a command
  line flag.

- In the generational mode, always collect the whole heap, as opposed
  to the bump pointer space only, if a collection is an explicit,
  native allocation-triggered or last attempt one.

Change-Id: I7a14a707cc47e6e3aa4a3292db62533409f17563
diff --git a/runtime/Android.mk b/runtime/Android.mk
index a3a8bec..f33635e 100644
--- a/runtime/Android.mk
+++ b/runtime/Android.mk
@@ -55,6 +55,7 @@
 	gc/collector/partial_mark_sweep.cc \
 	gc/collector/semi_space.cc \
 	gc/collector/sticky_mark_sweep.cc \
+	gc/gc_cause.cc \
 	gc/heap.cc \
 	gc/reference_queue.cc \
 	gc/space/bump_pointer_space.cc \