Add tvOS support

[darkwater]

Is your feature request related to a problem? Please describe.
I was trying to get Bevy to build for tvOS, and wgpu was the main source of errors.

Describe the solution you'd like
Add support for tvOS. Because it's so similar to iOS, this is mostly just changing cfg(target_os = "ios") into cfg(any(target_os = "ios", target_os = "tvos")). I already made some progress in this commit. I got a Bevy example to build and install, but it would immediately crash upon launch. See bevy#13058

Describe alternatives you've considered
Don't support tvOS.

Additional context
None

[jinleili]

The tvOS target is currently supported as part of the tier3 support level, and only supports no_std development

[darkwater]

It is indeed tier 3, but doesn't only support no_std. It seems this PR added libstd support. Either way, I was able to compile and install a Bevy example. It would crash at some iOS-specific functionality like trying to hide the status bar.

[VladasZ]

Hi @darkwater. I want to use wgpu with tvOS too.
I tried to add target = "tvos" like you did in my wgpu fork but now I get this link error:

Undefined symbols for architecture arm64:
  "_MTLCopyAllDevices", referenced from:
      metal::device::Device::all::h8012909a44210ef8 in libtest_game.a[3065](metal-d800b64461c8f587.metal.73d4cb208f4c1603-cgu.01.rcgu.o)
  "_objc_msg_lookup", referenced from:

Have you seen something like that?

Also I think that you should create a PR with your progress to the main wgpu repo. Yes it is not a full support for tvOS yet but it is a start and it will help other developers to progress from that and eventually add full tvOS support.

[madsmtm]

I'd recommend:

• macOS: cfg(target_os = "macos")
• iOS/tvOS/watchOS/visionOS/Mac Catalyst: cfg(all(target_vendor = "apple", not(target_os = "macos")))
• Metal: cfg(target_vendor = "apple")

At least until we get target_family = "darwin" or similar (see rust-lang/rust#100343).