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

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

coreutils/chgrp.c

@@ -16,7 +16,7 @@
 /* This is a NOEXEC applet. Be very careful! */
 
 
-int chgrp_main(int argc, char **argv);
+int chgrp_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int chgrp_main(int argc, char **argv)
 {
 	/* "chgrp [opts] abc file(s)" == "chown [opts] :abc file(s)" */