Added epoll and eventfd for Android

[YohDeadfall]

Fixes #3996.

I decided not to move check_shim into the implementations because all these functions are no more than just shims over syscall. Moreover, eventfd is even used by the syscall implementation.

[tiif]

If the intend is to allow eventfd on linux and android only, instead of removing the assert, maybe we could use

if !matches!(&*this.tcx.sess.target.os, "linux" | "android") {
     panic!("`eventfd` should not be called on {}", this.tcx.sess.target.os);
}

[YohDeadfall]

The thing here is that the eventfd implementation resides in the linux module, so I see no reason in that assertion. If it would be for any unix with some exceptions, then yes, there's a good reason to keep the assertion.

[YohDeadfall]

@RalfJung, what's your opinion?

[RalfJung]

Seems like maybe it's a good time to introduce a unix::linux_like module, and then put some things there that both unix::android and unix::linux can import and use as helpers.

I agree that in that module, if things work on all linux-like systems, they don't need a further assertion.

[RalfJung]

@rustbot author

[YohDeadfall]

@rustbot review

[bors]

☔ The latest upstream changes (presumably #4029) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #3939) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #4049) made this pull request unmergeable. Please resolve the merge conflicts.