libstdc++: Disable __cpp_lib_reflection for old CXX ABI

libstdc++: Disable __cpp_lib_reflection for old CXX ABI

Reflection currently doesn't work with -D_GLIBCXX_USE_CXX11_ABI=0.
The problem is that std::meta::exception currently uses under the
hood std::string and std::u8string and those aren't constexpr in
the old ABI.
While those members are in the standard exposition-only and so
we could make it to work by writing a custom class template that
just remembers const char{,8_t} * and size_t, there shouldn't be
many people trying to use C++26 features with the ABI that isn't
even compatible with C++11.

2026-01-20  Jakub Jelinek  <jakub@redhat.com>

	* include/bits/version.def (reflection): Add cxx11abi = yes;.
	* include/bits/version.h: Regenerate.