libstdc++: Embed a static copy of tzdata.zi

This adds a copy of the tzdata.zi file to the library, and allows
configuring to use it instead of a copy read from disk at runtime.
The content of the file is in the public domain, but will need to be
updated to the latest upstream file before making GCC releases.

libstdc++-v3/ChangeLog:

	* acinclude.m4 (GLIBCXX_ZONEINFO_DIR): Replace the
	--with-libstdcxx-zoneinfo-dir configure option with
	--with-libstdcxx-zoneinfo with yes/no/static choices as well as
	a directory.
	* config.h.in: Regenerate.
	* configure: Regenerate.
	* doc/xml/manual/configure.xml: Document configure option.
	* doc/html/manual/configure.html: Regenerate.
	* src/c++20/Makefile.am: Generate tzdata.zi.h header.
	* src/c++20/Makefile.in: Regenerate.
	* src/c++20/tzdb.cc (__gnu_cxx::zoneinfo_dir_override): Return a
	null pointer if no directory is configured.
	(zoneinfo_dir): Replace with ...
	(zoneinfo_file): New function.
	(tzdata_stream): New istream class.
	(remote_version, reload_tzdb): Use tzdata_stream.
	* testsuite/lib/libstdc++.exp (check_effective_target_tzdb):
	Check new _GLIBCXX_STATIC_TZDATA macro and ignore presence of
	tzdata.zi file in default location.
	* src/c++20/tzdata.zi: New file.