[5.1] Provide the host CPU to repository rules

[bazel-io]

Forked from #14685

[cameron-martin]

I'm confused. #14685 (comment) says that repository_ctx.os.name and repository_ctx.os.arch provide information about the host platform. However, this task is closed which implies there is a way of getting information about the execution platform. How do you do this?

[fmeum]

You can't - I think this issue's title was populated incorrectly when it was forked from #14685.

@meteorcloudy

[cameron-martin]

Makes sense, thanks!