Fix newlib to support <+-nn> timezone names (#7702)

Undoes the change in #7699 and fixed #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
  earlephilhower/newlib-xtensa#15
and undo the UNK changes used as an expedient workaround.

Author: earlephilhower