Reset LOCAL_2ND_ARCH_VAR_PREFIX before referencing it.

Change-Id: I9182ce1efe09609100d19c9fae2bc4fe7358dfc0
diff --git a/core/prebuilt.mk b/core/prebuilt.mk
index 33f5dc6..ba0e757 100644
--- a/core/prebuilt.mk
+++ b/core/prebuilt.mk
@@ -17,6 +17,7 @@
 my_skip_non_preferred_arch :=
 
 # check if first arch is supported
+LOCAL_2ND_ARCH_VAR_PREFIX :=
 include $(BUILD_SYSTEM)/module_arch_supported.mk
 ifeq ($(my_module_arch_supported),true)
 # first arch is supported