libstdc++: Test for tzdata.zi before fallback version files.

libstdc++: Test for tzdata.zi before fallback version files.

Several systems/distributions do not provide the raw tzdata.zi file in
their zoneinfo installation.  However, we might provide an alternate
installation path at configure time, so that we should check for the
tzdata.zi file first and then fall back to system-specific files like
+VERSION etc. on those systems.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
libstdc++-v3/ChangeLog:

	* src/c++20/tzdb.cc (remote_version): Look for the tzdata.zi
	file before falling back to system-specific ones on Darwin and
	BSD.