* TZ: help newlib parser
Timezones coded with numeric abbreviations <±nn>±nn<±nn>[±nn][,...] are incorrectly parsed
by newlib's TZ parser.
Replacing <±nn> occurences by UNK allows newlib's TZ parser to nicely interpret all timezones.
Detailed explanation in https://github.com/earlephilhower/newlib-xtensa/issues/12