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

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

archival/cpio.c

@@ -22,7 +22,7 @@
 #define CPIO_OPT_CREATE_LEADING_DIR     0x20
 #define CPIO_OPT_PRESERVE_MTIME         0x40
 
-int cpio_main(int argc, char **argv);
+int cpio_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int cpio_main(int argc, char **argv)
 {
 	archive_handle_t *archive_handle;