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.