Features that require `libc`

[madsmtm]

We kinda need to figure out a general policy on how we handle features that require libc (or at least just linking to it).

There are at least two things I'd like to do:

• Move MainThreadMarker into objc2 (with a more efficient implementation, namely libc::pthread_main_np)
  • Although really convenient, unsure if this is desired to have in objc2? I feel like it belongs in some lower-level library, though we don't have that, so I guess it'll have to be here.
• The things gated behind malloc support requires libc::free, I'd like to remove that feature gate to be able to verify more things in debug_assertions. Done in Work towards removing malloc_buf #547.

[silvanshade]

Is there any real downside to using it?

I'm using it in the cxx-auto stuff FWIW, although that would only affect the header-translator.

[madsmtm]

Downside is compilation speed, it's an extra dependency (with a build.rs script).

I'd be fine with using it in icrate, though also there probably behind a feature flag, again because it's an added dependency.

[madsmtm]

#616 changed the implementation of MainThreadMarker to use pthread_main_np and #625 moves it to objc2.