Timezones in <+-nn> format are not handled

[Andreas Merkle]

Posix timezone strings are in the format ABRVnn[ABRV[nn]][,...], e.g. 
GMT0BST,... is London TZ descriptor with two abbreviations GMT and BST.
ABRV means abbreviation. Such abbreviations are not defined for every 
timezone around the world.
According to https://data.iana.org/time-zones/theory.html if there is no 
common English abbreviations, use offsets like -05 and +0530 that are 
generated by zic's %z notation.
These numeric offsets are enclosed between <...>. For example, 
abbreviation for Sao Paulo TZ is <-03>3 (instead of e.g. valid SAOPAUL03).

A pull request for the newlib-xtensa version by earlephilhower fixes 
this: https://github.com/earlephilhower/newlib-xtensa/pull/14
I would like to provide the corresponding patch here if its ok?

Andi