[libc++][C++03] Add #if 0 to the experimental/ and ext/ headers as well (#119541)
This has already been done for the most headers in
https://github.com/llvm/llvm-project/pull/119234, but I
forgot to also do it for the experimental/ and ext/ headers.
This is part of https://discourse.llvm.org/t/rfc-freezing-c-03-headers-in-libc.