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

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

coreutils/uudecode.c

@@ -126,7 +126,7 @@ static void read_base64(FILE *src_stream, FILE *dst_stream)
 	}
 }
 
-int uudecode_main(int argc, char **argv);
+int uudecode_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int uudecode_main(int argc, char **argv)
 {
 	FILE *src_stream = stdin;