libstdc++: Use Clang attribute instead of __constinit

Clang doesn't support the __constinit extension that we use pre-C++20,
but it does have its own equivalent attribute that can be used instead.

This makes it a little easier to use Clang to build libstdc++ (which
isn't supported. but is sometimes attempted for esoteric targets).

libstdc++-v3/ChangeLog:

	* src/c++11/cxx11-ios_failure.cc (__constinit): Define as
	equivalent attribute for Clang.
	* src/c++11/future.cc (__constinit): Likewise.
	* src/c++11/system_error.cc (__constinit): Likewise.
	* src/c++17/memory_resource.cc (__constinit): Likewise.