libstdc++: Disable unwanted code for --with-libstdcxx-zoneinfo=no

libstdc++: Disable unwanted code for --with-libstdcxx-zoneinfo=no

This allows most of the tzdb functionality to be disabled by
configuring with --with-libstdcxx-zoneinfo=no. This might be desirable
for small targets that don't need the time zone support.

libstdc++-v3/ChangeLog:

	* src/c++20/tzdb.cc (TZDB_DISABLED): Disable all code for
	loading tzdb.
	* testsuite/std/time/tzdb/leap_seconds.cc: Require tzdb
	effective target.
	* testsuite/std/time/tzdb_list/1.cc: Likewise.