Fix newlib to support <+-nn> timezone names

Undoes the change in esp8266#7699 and fixed esp8266#7690 root cause.

Newlib did not support timezone names of the form "<[+-]?[0-9]+>" and
would parse the offset using the <name>.

Fix newlib tzset parser with
  earlephilhower/newlib-xtensa#14
and undo the UNK changes used as an expedient workaround.

Author: earlephilhower