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)))