libstdc++: Add c++2a <syncstream>

libstdc++: Add c++2a <syncstream>

libstdc++-v3/ChangeLog:
	* doc/doxygen/user.cfg.in (INPUT): Add new header.
	* include/Makefile.am (std_headers): Add new header.
	* include/Makefile.in: Regenerate.
	* include/precompiled/stdc++.h: Include new header.
	* include/std/syncstream: New header.
	* include/std/version: Add __cpp_lib_syncbuf.
	* testsuite/27_io/basic_syncbuf/1.cc: New test.
	* testsuite/27_io/basic_syncbuf/2.cc: Likewise.
	* testsuite/27_io/basic_syncbuf/basic_ops/1.cc:
	Likewise.
	* testsuite/27_io/basic_syncbuf/requirements/types.cc:
	Likewise.
	* testsuite/27_io/basic_syncbuf/sync_ops/1.cc:
	Likewise.
	* testsuite/27_io/basic_syncstream/1.cc: Likewise.
	* testsuite/27_io/basic_syncstream/2.cc: Likewise.
	* testsuite/27_io/basic_syncstream/basic_ops/1.cc:
	Likewise.
	* testsuite/27_io/basic_syncstream/requirements/types.cc:
	Likewise.