Conditionally create the final lib after download

Author: alexcrichton

Makefile

@@ -592,6 +592,9 @@ $(BUILTINS_LIB_PATH):
 	mkdir -p $(BUILTINS_LIB_DIR)
 	curl -sSfL $(BUILTINS_URL) | \
 		tar xzf - -C $(BUILTINS_LIB_DIR) --strip-components 1
+	if [ ! -f $(BUILTINS_LIB_PATH) ]; then \
+	  mv $(BUILTINS_LIB_DIR)/*.a $(BUILTINS_LIB_PATH); \
+	fi
 endif
 
 # TODO: Specify SDK version, e.g. libc.so.wasi-sdk-21, as SO_NAME once `wasm-ld`