v8: support multilib builds

Allow building for multiple architectures at the same time:
Use $(local-generated-sources-dir) instead of $(local-intermediates-dir)
Set LOCAL_CFLAGS_arch instead of ifeq ($(TARGET_ARCH),arch)
Use LOCAL_MODULE_TARGET_ARCH_WARN to restrict which architectures
attempt to build, and print a warning on the rest.

Change-Id: I66dffb36557aa47c92a1ae158683a27e9ca83e53
diff --git a/Android.d8.mk b/Android.d8.mk
index 0f74eb5..f512e75 100644
--- a/Android.d8.mk
+++ b/Android.d8.mk
@@ -19,4 +19,6 @@
 
 LOCAL_C_INCLUDES += $(LOCAL_PATH)/include
 
+LOCAL_MODULE_TARGET_ARCH_WARN := $(V8_SUPPORTED_ARCH)
+
 include $(BUILD_EXECUTABLE)