js-on-sx: js-neg preserves IEEE-754 negative zero

JS -0 was returning rational integer 0; the (- 0 x) form loses the
sign-of-zero. Switched js-neg to (* -1 (exact->inexact (js-to-number a))),
which produces a float and preserves -0.0. Now 1/(-0) === -Infinity
and Math.asinh(-0) preserves the sign as required by the spec.
built-ins/Math: 41/45 → 42/45. conformance.sh: 148/148.

lib/js/test262-scoreboard.json

@@ -56,6 +56,6 @@
     ]
   ],
   "pinned_commit": "d5e73fc8d2c663554fb72e2380a8c2bc1a318a33",
-  "elapsed_seconds": 420.7,
+  "elapsed_seconds": 250.0,
   "workers": 1
 }