Set build_platform for aid in cross compiling

[isuruf]

No description provided.

[isuruf]

Ping @mingwandroid

[jjhelmus]

Is the intention to have build_platform be a this be a pre-processor selector or an environment variable exported into the build environment?

This PR does the former but the later seems like a useful feature.

[isuruf]

I wanted the former. For example along with #4011,

package:
  name: icu
requirements:
  build:
    - icu    # [build_platform != target_platform]
  run:
  host:

I can add this as a environment variable too.

[jjhelmus]

Thanks @isuruf. These will need to be added to the docs at some point. I created #4018 for this.

[github-actions]

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