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.

dirent contains a flexible array member, so don't test its sizeof, don't
allow it to be copied, and don't represent it with an artificial size.

Also, locale_t is a typedef for a pointer to an opaque struct, so
represent it that way explicitly.

Rustc is changing it to be target_os; for now, recognize both.

defined for 32-bit Solarish targets.  Since rust doesn't currently
support 32-bit Solarish targets, the line is both unnecessary and wrong.

Added recently and will be used in libstd!

Now that wasi is in nightlies, we can run it on PRs!

This target is [being proposed][LINK] int he rust-lang/rust repository
and this is intended to get coupled with that proposal. The definitions
here all match the upstream reference-sysroot definitions and the
functions all match the reference sysroot as well. The linkage here is
described more in detail on the Rust PR itself, but in general it's
similar to musl.

Automatic verification has been implemented in the same manner as other
targets, and it's been used locally to develop this PR and catch errors
in the bindings already written (also to help match the evolving sysroot
of wasi). The verification isn't hooked up to CI yet though because
there is no wasi target distributed via rustup just yet, but once that's
done I'll file a follow-up PR to execute verification on CI.

[LINK]: