[#1049] Fix (u)int64 returned from callout is truncated to 32 bits

• int64 bug: This only affects only values returned as xxx_(u)int64_t, not via a xxx_(u)int64_t* pointer. Hence, it only affects values returned explicitly, since all values returned via parameters must use pointer-type parameters. This bug was caught by test xcall/retvals (YDBTest!1772 (merged))