math: use '#pragma STDC FENV_ACCESS ON' when fenv is accessed

src/math/fmal.c

@@ -162,6 +162,7 @@ static inline struct dd dd_mul(long double a, long double b)
  */
 long double fmal(long double x, long double y, long double z)
 {
+	#pragma STDC FENV_ACCESS ON
 	long double xs, ys, zs, adj;
 	struct dd xy, r;
 	int oround;