fast IDate/Time methods for UTC can be extended to when we can infer tz is UTC

Currently we test:

```
if (inherits(x, "POSIXct") && identical(attr(x, "tzone", exact = TRUE), "UTC")) 
```

However, if `'tzone'` attribute is missing and `Sys.timezone()` is UTC (actually, `%chin% c('GMT', 'UTC', 'Etc/UTC', 'Etc/GMT', 'GMT-0', 'GMT+0', 'GMT0')` is fine), we can do the same shortcut