Add cfg_attr

 - Rename `wasm32-unknown-wasi` to `wasm32-wasi`.
 - `__wasilibc_rmfileat` was renamed to `__wasilibc_unlinkat`
 - Add bindings for a few more functions and typedefs.

See https://github.com/rust-lang/rust/pull/60775 .

[breaking change] PTRACE_GETFPXREGS and PTRACE_SETFPXREGS were incorrectly re-exported on arm but they are x86 specific

This is necessary to set socket options on the Nintendo Switch, which uses a socket implementation directly ported from FreeBSD.

fd_set isn't automatically copyable. While it will be possible to fix
that, for now just remove these so that they don't block other changes.