Add rustc-dep-of-std feature

[josephlr]

This is how libc and hashbrown support being dependancies of libstd.
This allows getrandom to use core normally, and use the sysroot core
when building libstd.

This PR and #58 should be enough to get rust-lang/rust#62082 building without errors.

[newpavlov]

You probably also should remove uses of --all-features from CI tests.

I am not familiar with the details of how std is built, so I hope someone more knowledgeable will confirm, that it's indeed a correct approach in our case.

cc @alexcrichton

[josephlr]

You probably also should remove uses of --all-features from CI tests.

Done

I am not familiar with the details of how std is build, so I hope someone more knowledgeable will confirm, that it's indeed a correct approach in our case.

I know we need at least this to properly use the core and compiler_builtins libraries in the sysroot. We may also need additional functionality, I'm not sure.

[alexcrichton]

I can confirm that for building as part of the standard library the Cargo.toml bits are correct and would be required.

[josephlr]

This should be ready for merging, I updated the Redox comment to better explain the issue in #79.

I also confirmed that this PR + #58 + a modified version of @newpavlov's rust-lang/rust#62082 is now enough to bootstrap libstd without errors.

EDIT: I should also note that this does not depend on #58