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

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

coreutils/sort.c

@@ -274,7 +274,7 @@ static unsigned str2u(char **str)
 }
 #endif
 
-int sort_main(int argc, char **argv);
+int sort_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int sort_main(int argc, char **argv)
 {
 	FILE *fp, *outfile = stdout;