Suppress a compiler warning generated by Clang.

Clang doesn't like it if you bitshift negative numbers, as the behaviour
of it is undefined. Solve this by first shifting, followed by negating.
This was fixed similarly in FreeBSD.

src/s_exp2.c

@@ -375,14 +375,14 @@ exp2(double x)
 	/* Compute r = exp2(y) = exp2t[i0] * p(z - eps[i]). */
 	t = tbl[i0];		/* exp2t[i0] */
 	z -= tbl[i0 + 1];	/* eps[i0]   */
-	if (k >= -1021 << 20)
+	if (k >= -(1021 << 20))
 		INSERT_WORDS(twopk, 0x3ff00000 + k, 0);
 	else
 		INSERT_WORDS(twopkp1000, 0x3ff00000 + k + (1000 << 20), 0);
 	r = t + t * z * (P1 + z * (P2 + z * (P3 + z * (P4 + z * P5))));
 
 	/* Scale by 2**(k>>20). */
-	if(k >= -1021 << 20) {
+	if(k >= -(1021 << 20)) {
 		if (k == 1024 << 20)
 			return (r * 2.0 * 0x1p1023);
 		return (r * twopk);