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

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

findutils/xargs.c

@@ -373,7 +373,7 @@ enum {
 	USE_FEATURE_XARGS_SUPPORT_TERMOPT(     "x") \
 	USE_FEATURE_XARGS_SUPPORT_ZERO_TERM(   "0")
 
-int xargs_main(int argc, char **argv);
+int xargs_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int xargs_main(int argc, char **argv)
 {
 	char **args;