ld128: fix coshl returning 1+x instead of 1 for tiny |x|

The early-exit branch in coshl for tiny |x| returned `w = 1 + expm1l(x)`,
which is ~1+x for representable small x, not 1. Reproduced on AArch64
(112-bit mantissa) per #285:

  coshl(0x1p-72) = 0x1.000000000000000001p+0  (should be 1.0)

cosh(x) = 1 + x^2/2 + O(x^4); for any |x| < 2^-71 the x^2/2 term is
below ulp(1) = 2^-112, so returning exactly 1 is correctly rounded.

Also fix the threshold comment: 0x3fb80000 is exp 0x3fb8 - bias 0x3fff
= -71, not -116.

Closes #285.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ViralBShah

ld128/e_coshl.c

@@ -75,8 +75,8 @@ coshl(long double x)
     {
       t = expm1l (u.value);
       w = one + t;
-      if (ex < 0x3fb80000) /* |x| < 2^-116 */
-	return w;		/* cosh(tiny) = 1 */
+      if (ex < 0x3fb80000) /* |x| < 2^-71 */
+	return one;		/* cosh(tiny) = 1 */
 
       return one + (t * t) / (w + w);
     }