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

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

editors/vi.c

@@ -311,7 +311,7 @@ static void write1(const char *out)
 	fputs(out, stdout);
 }
 
-int vi_main(int argc, char **argv);
+int vi_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int vi_main(int argc, char **argv)
 {
 	int c;