Add support for watchOS.

[pixlwave]

This PR adds support to parking_lot for watchOS. This can be tested by building on a mac with the following command when the watchOS SDK is installed through Xcode:

cargo build -Zbuild-std --target=arm64_32-apple-watchos

The support simply mirrors macOS and iOS support.

Note: I've also got a watchos-0.8 branch with the changes cherry picked onto backport-core-0.8, I'm not exactly sure how backports are handled.

[Amanieu]

Would it make sense to add tvos as well?

Regarding backports, I prefer not to unless there is a very good reason. Generally speaking the only supported version of this crate is the latest published version. If you need support for watchOS then you should upgrade your dependencies to the latest version.

[pixlwave]

Hey, sorry for the slow reply. It would be nice to add tvos too, but it doesn't have support for std yet (although that seems to be in progress). From what I can tell parking_lot doesn't work for no_std targets, although I'm still fairly new to Rust so could easily be missing something here.

Understood on the backports, makes total sense to me :)

[pixlwave]

Wooo, thanks 🥳