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/runtime.sx

@@ -1453,7 +1453,7 @@
 
 (define js-pow (fn (a b) (pow (js-to-number a) (js-to-number b))))
 
-(define js-neg (fn (a) (- 0 (js-to-number a))))
+(define js-neg (fn (a) (* -1 (exact->inexact (js-to-number a)))))
 
 (define js-pos (fn (a) (js-to-number a)))
 

lib/js/test262-scoreboard.json

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

lib/js/test262-scoreboard.md

@@ -1,7 +1,7 @@
 # test262 scoreboard
 
 Pinned commit: `d5e73fc8d2c663554fb72e2380a8c2bc1a318a33`
-Wall time: 420.7s
+Wall time: 250.0s
 
 **Total:** 77/99 runnable passed (77.8%). Raw: pass=77 fail=16 skip=1 timeout=6 total=100.