Publish x86_arm/crossgen during Linux/arm build

[echesakov]

This PR adds compiling and publishing of x86_arm/crossgen (so called, cross-crossgen) to Linux/arm official build

Merge ONLY AFTER dotnet/dotnet-buildtools-prereqs-docker#22 is resolved and official build images are updated

[RussKeldorph]

Is this path correct?

[sdmaclea]

Can someone explain this to me? Why do we want to cross-crossgen?

[sdmaclea]

Why do we need a single docker image with both x86 and arm rootfs?

[sdmaclea]

Likewise, why don't we need arm64 in the same image?

[echesakov]

@sdmaclea I can answer your first two questions:
We want to build crosscomponents (which include x86_arm crossgen or cross-crossgen and x86_arm clrjit) in the same docker image during build.sh. The idea of cross-crossgen is to publish from developer machine for Linux/arm. x86 rootfs is needed to build x86_arm crossgen inside x64 docker container.

I also work on getting x64_arm crossgen #16513 and when this work is done there will be no need in x86 rootfs.

For x64_arm64 crossgen we don't need second rootfs. @jashook might know why we have two separate arm and arm64 docker images.

[echesakov]

We publish this cross-component as NuGet package, so it can be consumed by other DotNet components to produce Ready2Run images. For example, coreclr build will do this for System.Private.CoreLib.dll.

[sdmaclea]

So instead of using an x86 ubuntu container with an arm rootfs running on x64 host.
You are running an x64 ubuntu container with two rootfs.

Thanks. Arm64 doesn't have these complications.

[jashook]

Correct arm64 does not need these complications because we can cross target from x64.

[jashook]

This is now unblocked. Docker image to use is: microsoft/dotnet-buildtools-prereqs:ubuntu-16.04-cross-e435274-20180317125354

[echesakov]

@jashook Updated the docker image tag. PTAL

[jashook]

Stil lgtm, @mmitche for another reviewer.