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

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

miscutils/adjtimex.c

@@ -43,7 +43,7 @@ static const char *const ret_code_descript[] = {
 	"clock not synchronized"
 };
 
-int adjtimex_main(int argc, char **argv);
+int adjtimex_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int adjtimex_main(int argc, char **argv)
 {
 	enum {