Go back to -O2.

Stack space suffers otherwise.

Change-Id: I890a9714acd8500981c3d41df0f5f7fd63d48f8f
diff --git a/build/Android.common_build.mk b/build/Android.common_build.mk
index 70b3448..3000cdf 100644
--- a/build/Android.common_build.mk
+++ b/build/Android.common_build.mk
@@ -228,7 +228,7 @@
 
 # Cflags for debug ART and ART tools.
 art_debug_cflags := \
-  -O0 \
+  -O2 \
   -DDYNAMIC_ANNOTATIONS_ENABLED=1 \
   -DVIXL_DEBUG \
   -UNDEBUG