Add flag to disable 64-bit driver.

bug 17428047

Change-Id: I888dd310449bf9079e352cb4aef31344c05b0ba8
diff --git a/Android.mk b/Android.mk
index e8ec005..b14d2d8 100644
--- a/Android.mk
+++ b/Android.mk
@@ -10,6 +10,10 @@
   rs_base_CFLAGS += -DOVERRIDE_RS_DRIVER=$(OVERRIDE_RS_DRIVER)
 endif
 
+ifneq ($(DISABLE_RS_64_BIT_DRIVER),)
+  rs_base_CFLAGS += -DDISABLE_RS_64_BIT_DRIVER
+endif
+
 ifeq ($(RS_FIND_OFFSETS), true)
   rs_base_CFLAGS += -DRS_FIND_OFFSETS
 endif