gbasm: truncate file at end of output

When overwriting an old output file, this will delete the extra data
if the new output is shorter than the old output.

src/apps/gbasm.c

@@ -39,7 +39,7 @@ int main(int argc, char **argv)
 				close(out_fd);
 			}
 
-			out_fd = open(optarg, O_WRONLY | O_CREAT);
+			out_fd = open(optarg, O_WRONLY | O_CREAT | O_TRUNC);
 			if (out_fd < 0) {
 				fprintf(stderr, "failed to open output file %s\n", optarg);
 				exit(1);