[release-branch.go1.15] time: correct unusual extension string cases

This fixes two uncommon cases.

First, the tzdata code permits timezone offsets up to 24 * 7, although
the POSIX TZ parsing does not. The tzdata code uses this to specify a
day of week in some cases.

Second, we incorrectly rejected a negative time offset for when a time
zone change comes into effect.

For #44385
Fixes #44617

Change-Id: I5f2efc1d385e9bfa974a0de3fa81e7a94b827602
Reviewed-on: https://go-review.googlesource.com/c/go/+/296392


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
(cherry picked from commit d9fd38e6)
Reviewed-on: https://go-review.googlesource.com/c/go/+/297229