[JS] UInt inconsistent result across targets

The following code will produce ```-1073202714836107800``` in JS target , but ```573931030``` in other ones
```haxe
var m:UInt = 0xc6a4a793;
var h:UInt = 1115261778;
h = h*m;
trace("h: "+h);
```
https://try.haxe.org/#8C3Cb55F

Start by reproducing the code from the issue in the linked try.haxe.org example, comparing the JS target with the other targets. Trace the UInt multiplication and result formatting to determine where the targets diverge; done means the example produces consistent results across targets.

Written by the indexing model from the issue text.