libstdc++: Fix --with-default-libstdcxx-abi=gcc4-compatible build

libstdc++: Fix --with-default-libstdcxx-abi=gcc4-compatible build

When building src/c++20/tzdb.cc we currently get a build error for
--with-default-libstdcxx-abi=gcc4-compatible because std::chrono::tzdb
and related types are not declared for the gcc4-compatible ABI (unless
--disable-libstdcxx-dual-abi is also used, so that the gcc4-compatible
ABI is the only one built).

Define _GLIBCXX_USE_CXX11_ABI in tzdb.cc so that for a dual-abi build we
always build it for the cxx11 ABI.

libstdc++-v3/ChangeLog:

	* src/c++20/tzdb.cc (_GLIBCXX_USE_CXX11_ABI): Define to 1.