[Libc] Fixes the declaration of __fc_nan and names ensures clauses.

share/libc/math.h

@@ -762,16 +762,16 @@ extern int __finite(double d);
   @*/
 
 /*@
-  ensures \eq_float(\result,\plus_infinity);
+  ensures result_is_infinity: \is_plus_infinity(\result);
   assigns \result \from \nothing;
   @*/
 extern const float __fc_infinity(int x);
 
 /*@
-  ensures \is_NaN(\result);
+  ensures result_is_nan: \is_NaN(\result);
   assigns \result \from \nothing;
   @*/
-extern const float __fc_nan();
+extern const float __fc_nan(int x);
 
 
 #define INFINITY __fc_infinity(0)