Make Crankshaft configurable at compile-time.

Add a flag in Android.mk to enable crankshaft in V8.

Note that Crankshaft and snapshotting cannot currently be used
at the same time. For now, leave Crankshaft disabled by default.

Change-Id: I84fcc02882e1b16cd0bc783d778ffbcc4531c868
diff --git a/CleanSpec.mk b/CleanSpec.mk
index 7a097df..49ab2c1 100644
--- a/CleanSpec.mk
+++ b/CleanSpec.mk
@@ -49,6 +49,10 @@
 $(call add-clean-step, rm -rf $(OUT_DIR)/host/$(HOST_OS)-$(HOST_ARCH)/obj/EXECUTABLES/mksnapshot_intermediates)
 
 
+$(call add-clean-step, rm -rf $(PRODUCT_OUT)/obj/STATIC_LIBRARIES/libv8_intermediates)
+$(call add-clean-step, rm -rf $(OUT_DIR)/host/$(HOST_OS)-$(HOST_ARCH)/bin/mksnapshot)
+$(call add-clean-step, rm -rf $(OUT_DIR)/host/$(HOST_OS)-$(HOST_ARCH)/obj/EXECUTABLES/mksnapshot_intermediates)
+
 # ************************************************
 # NEWER CLEAN STEPS MUST BE AT THE END OF THE LIST
 # ************************************************