time: add support for parsing timezones denoted by sign and offset

time: add support for parsing timezones denoted by sign and offset

IANA Zoneinfo does not provide names for all timezones. Some are denoted
by a sign and an offset only. E.g: Europe/Turkey is currently +03 or
America/La_Paz which is -04 (https://data.iana.org/time-zones/releases/tzdata2018c.tar.gz)

Fixes #24071

Change-Id: I9c230a719945e1263c5b52bab82084d22861be3e
Reviewed-on: https://go-review.googlesource.com/98157
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>