[tz] Etc/GMT+nn and Etc/GMT-nn not working since zic 2021d

[Peter Krefting]

Hi!

I am creating timezone data for an embedded device, and with space as a
premium I run zic with the -r parameter to cut off all time change
information from before the software release date, as this is not relevant
for the system.

Since commit 5f06f9a3182d2c2942be1b678e4defa9c7400653 (included in 2021d),
this causes the Etc/GMT+xx and Etc/GMT-xx timezones not to work, they always
report GMT ("-00"):

   ~ # TZ=Europe/Oslo date
   Mon Mar 21 13:08:32 CET 2022
   ~ # TZ=Europe/London date
   Mon Mar 21 12:08:36 GMT 2022
   ~ # TZ=Etc/GMT+1 date
   Mon Mar 21 12:08:43 -00 2022
   ~ # TZ=Etc/GMT+2 date
   Mon Mar 21 12:08:45 -00 2022

This is particularly severe for our devices since previous versions of our
software only supported UTC offsets, and these are automatically updated to
these GMT+xx and GMT-xx timezones on update, so many of our customers are
expected to have these zones configured.

We are running with "-r cutoff -b fat", where cutoff depends on the release
date, and the embedded software currenlty is using GNU libc 2.20.

Reverting 5f06f9a3182d2c2942be1b678e4defa9c7400653 fixes the issue for us,
but is probably not the correct approach.

-- 
\\// Peter - http://www.softwolves.pp.se/