Move std::filesystem directory iterators and operations from
libstdc++fs.a to main libstdc++ library. These components have many
dependencies on OS support, which is not available on all targets. Some
additional autoconf checks and conditional compilation is needed to
ensure the files will build for all targets. Previously this code was
not compiled without --enable-libstdcxx-filesystem-ts but the C++17
components should be available for all hosted builds.

The tests for these components no longer need to link to libstdc++fs.a,
but are not expected to pass on all targets. To avoid numerous failures
on targets which are not expected to pass the tests (due to missing OS
functionality) leave the dg-require-filesystem-ts directives in place
for now. This will ensure the tests only run for builds where the
filesystem-ts library is built, which presumably means some level of OS
support is present.


Tested x86_64-linux (old/new string ABIs, 32/64 bit), x86_64-w64-mingw32.

Committed to trunk.