gcc 4.4 does not have __builtin_unreachable

For avr32 we still use gcc 4.4 compiler, so exlude the usage
of __builtin_unreachable here.

ldso/include/dl-syscall.h

@@ -51,7 +51,7 @@ extern int _dl_errno;
 static __always_inline attribute_noreturn __cold void _dl_exit(int status)
 {
 	INLINE_SYSCALL(_dl_exit, 1, status);
-#if defined __GNUC__
+#if defined __GNUC__ && !__GNUC_PREREQ (4, 4)
 	__builtin_unreachable(); /* shut up warning: 'noreturn' function does return*/
 #else
 	while (1);