libstdc++: Add workaround for old tzdata.zi files

libstdc++: Add workaround for old tzdata.zi files

The tzdata.zi file in the RHEL 6 tzdata-2018e-3.el6 package (with
version "unknown") does not conform to the current rules described in
the zic(8) man page. Specifically, a Rule name must not start with the
character '+' in the current rules, but the older tzdata.zi file
used "+" as the name of rules for the "Europe/Sofia" zone.

Add a special case to the logic that detects whether a RULES field
refers to a named rule or is an offset from standard time. For a string
matching exactly "+" treat it as a named Rule, but for any other string
starting with '+' treat it as an offset.

libstdc++-v3/ChangeLog:

	* src/c++20/tzdb.cc (operator>>(istream&, ZoneInfo&)): Allow
	rules named "+" for compatibility with older tzdata.zi files.