Real "LOCAL_MULTILIB := both" for prebuilts

This uses the fact that unsetting LOCAL_MULTILIB equals "either".
It's useful to build for both 32-bit and 64-bit in the same prebuilt
module definition.

Bug: 13751317
Change-Id: I4f1625a83e13f22f807039afebae73f69ed35918
diff --git a/core/prebuilt_internal.mk b/core/prebuilt_internal.mk
index 089cc63..f63d291 100644
--- a/core/prebuilt_internal.mk
+++ b/core/prebuilt_internal.mk
@@ -19,13 +19,19 @@
 # Not much sense to check build prebuilts
 LOCAL_DONT_CHECK_MODULE := true
 
+my_32_64_bit_suffix := $(if $($(LOCAL_2ND_ARCH_VAR_PREFIX)$(my_prefix)IS_64_BIT),64,32)
+
 ifdef LOCAL_PREBUILT_MODULE_FILE
   my_prebuilt_src_file := $(LOCAL_PREBUILT_MODULE_FILE)
 else
   ifdef LOCAL_SRC_FILES_$($(my_prefix)$(LOCAL_2ND_ARCH_VAR_PREFIX)ARCH)
     my_prebuilt_src_file := $(LOCAL_PATH)/$(LOCAL_SRC_FILES_$($(my_prefix)$(LOCAL_2ND_ARCH_VAR_PREFIX)ARCH))
   else
-    my_prebuilt_src_file := $(LOCAL_PATH)/$(LOCAL_SRC_FILES)
+    ifdef LOCAL_SRC_FILES_$(my_32_64_bit_suffix)
+      my_prebuilt_src_file := $(LOCAL_PATH)/$(LOCAL_SRC_FILES_$(my_32_64_bit_suffix))
+    else
+      my_prebuilt_src_file := $(LOCAL_PATH)/$(LOCAL_SRC_FILES)
+    endif
   endif
 endif