Create a POSIX `<nl_types.h>` header with `catopen`, `catclose`, and
`catgets` function declarations.
Provide the stub/placeholder implementations which always return error.
This is consistent with the way
locales are currently (un-)implemented in llvm-libc.
Notably, providing `<nl_types.h>` fixes the last remaining issue with
building libc++ against llvm-libc
(on certain configuration of x86_64 Linux) after disabling threads and
wide-characters in libc++.
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