Add support for x86_64-unknown-none targets

[jblebrun]

By default, no sysroot is used.

This is useful for bare-metal targets, firmwares, etc.

We are actively using this patch in https://github.com/project-oak/oak

[fmeum]

Could you look into the CI failures?

[jblebrun]

Could you look into the CI failures?

Taking a look. It looks like a lot of the failures are due to wasm failures, because of missing headers. I wonder if it's due to the sysroot change I made.

I can't reproduce the failure locally, so I created commenting this out and pushing again.

[jblebrun]

I can't reproduce the failure locally, so I created commenting this out and pushing again.

Update: I found a local repro. I will figure out what's happening and push a new update. So you can hold off on enabling the current CI run.

[jblebrun]

Ok, I figured it out. Checking on os == "none" alone was also allowing wasm32|64-none-none to pass.

To fix it, I've created a list of target pairs that should support no-sysroot.

It seems it should be theoretically possible to support wasm-none as well, but that might be a deeper change?

[jblebrun]

@fmeum This PR is ready for another test pass whenever you get a chance to click the button.

[jblebrun]

Thanks!