support setting host_platform explicitly

[wolfv]

@isuruf I started a PR here to add --host-platform to the arguments.

The current logic is:

The host_platform is defaulted to Platform::current().

If target_platform is unset, then we initialize it to the value of host_platform.

If target_platform is set to something (noarch is forbidden to set btw), then host_platform is set to this value too, if it is not set explicitly (so the current logic continues to work).

Would be great if you let me know if that makes sense to you and what use-cases you had in mind.

[isuruf]

Sounds good to me. If both host_platform and target_platform are set, they are not changed right?

The use I have is for the compilers where we would build gcc_{{ target_platform }} which is going to run on host_platform, but produce executables to be run on target_platform. This compiler is built on build_platform.

[wolfv]

Let me try this use case locally to see what it does :) I'll also add host_platform to the available variables during rendering.