libstdc++: Fix __cpp_lib_chrono for old std::string ABI

libstdc++: Fix __cpp_lib_chrono for old std::string ABI

The <chrono> header is incomplete for the old std::string ABI, because
std::chrono::tzdb is only defined for the new ABI. The feature test
macro advertising full C++20 support should not be defined for the old
ABI.

libstdc++-v3/ChangeLog:

	* include/bits/version.def (chrono): Add cxx11abi = yes.
	* include/bits/version.h: Regenerate.
	* testsuite/std/time/syn_c++20.cc: Adjust expected value for
	the feature test macro.