genzabbrs.go uses whatever zoneinfo database available on the system.
This makes genzabbrs.go output change from system to system. Adjust
go:generate line to always use $GOROOT/lib/time/zoneinfo.zip, so it
does not matter who runs the command.

Also move go:generate line into zoneinfo.go, so it can be run
on Unix (see #16368 for details).

Fixes #15802.

Change-Id: I8ae4818aaf40795364e180d7bb4326ad7c07c370
Reviewed-on: https://go-review.googlesource.com/27832Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>