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

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

miscutils/strings.c

@@ -16,7 +16,7 @@
 #define PRINT_OFFSET	4
 #define SIZE			8
 
-int strings_main(int argc, char **argv);
+int strings_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int strings_main(int argc, char **argv)
 {
 	int n, c, status = EXIT_SUCCESS;