Help newlib TZ parser (#7699)

* 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 earlephilhower/newlib-xtensa#12

Author: d-a-v