ref #16054 undefine some stuff in JS backend (#16070)

lib/system.nim

@@ -1897,6 +1897,7 @@ else:
 # however, stack-traces are available for most parts
 # of the code
 
+when notJSnotNims:
   var
     globalRaiseHook*: proc (e: ref Exception): bool {.nimcall, benign.}
       ## With this hook you can influence exception handling on a global level.