Add __attribute__ ((returns_twice)) to the ia64_getcontext prototype

so that gcc will make the right arrangements and avoid an illegal
instruction during call-with-current-continuation.

libguile/continuations.c

@@ -120,6 +120,10 @@ struct rv
   long retval;
   long first_return;
 };
+
+#ifdef __GNUC__
+__attribute__ ((returns_twice))
+#endif /* __GNUC__ */
 extern struct rv ia64_getcontext (ucontext_t *) __asm__ ("getcontext");
 #endif /* __ia64__ */