add -fvisibility=hidden to CC flags, mark XXX_main functions

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

archival/rpm2cpio.c

@@ -49,7 +49,7 @@ static void skip_header(int rpm_fd)
 }
 
 /* No getopt required */
-int rpm2cpio_main(int argc, char **argv);
+int rpm2cpio_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int rpm2cpio_main(int argc, char **argv)
 {
 	struct rpm_lead lead;