Support non-POSIX TZ strings

Hello,

I stumbled upon an issue with some TZ strings not handled as expected by newlib's tzset() function.
The tzset functions expects the string stored in the TZ environment variable to follow the POSIX format as described here: https://sourceware.org/newlib/libc.html#tzset <https://sourceware.org/newlib/libc.html#tzset> (or https://www.gnu.org/software/libc/manual/html_node/TZ-Variable.html <https://www.gnu.org/software/libc/manual/html_node/TZ-Variable.html>).

However, the glibc implementation extends the format and additionally allows ‘<[+|-]hh[:mm[:ss]]>’ in the format (compare https://www.man7.org/linux/man-pages/man3/tzset.3.html <https://www.man7.org/linux/man-pages/man3/tzset.3.html>). It seems like the timezone database (zoneinfo) provided by the IANA (https://www.iana.org/time-zones <https://www.iana.org/time-zones>) adopted that format; or at least the zic compiler generates these strings in the zoneinfo files for most systems.

That leads to the timezone for "America/Argentina/Buenos_Aires” to be "<-03>3”, as can be seen in this dump https://raw.githubusercontent.com/nayarsystems/posix_tz_db/master/zones.csv <https://raw.githubusercontent.com/nayarsystems/posix_tz_db/master/zones.csv> or a linux system: `tail -n 1 /usr/share/zoneinfo/America/Argentina/Buenos_Aires`.

Some more background information can be found here https://github.com/esp8266/Arduino/issues/8423 <https://github.com/esp8266/Arduino/issues/8423> and here https://github.com/esp8266/Arduino/issues/7690 <https://github.com/esp8266/Arduino/issues/7690>.

One way to approach this is for the user to just replace the incompatible part of the string with a valid timezone identifier, as proposed by https://github.com/esp8266/Arduino/pull/7699 <https://github.com/esp8266/Arduino/pull/7699>.
Since the timezone identifier (e.g. `PST`, `PDT`, `CET`, …) is not really used elsewhere by newlib, this should not be a problem, as far as I can imagine.

On the other hand, some ports implemented a proper parsing: https://github.com/earlephilhower/newlib-xtensa/pull/14 <https://github.com/earlephilhower/newlib-xtensa/pull/14>.

Now my question is whether the extended format should be support by newlib? Is this desired behaviour and would you accept code contributions for that matter?


Kind Regards
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jdoubleu