Add build flags to allow overriding platforms used in transitions

[brentleyjones]

This allows people to use custom versions of the platforms, including child platforms of the default ones, which allows for custom additional constraints and platform properties.

[brentleyjones]

I now think they should have a suffix of _default instead of a prefix. Then it reads more like "this is cpu ios_sim_arm's default platform".

And "default" is probably wrong, because the default value for them is the default, but you are mostly likely changing it away from the default if you use the flag.

So maybe it should actually be in a different package (root?) and named something like @apple_support//:ios_sim_arm_platform (which admittedly is a little confusing with the platforms package existing as well). This makes sense to me though because when set by a user they are expressing what is changing (--@apple_support//:ios_sim_arm_platform=//custom_platforms:ios_sim or their default value --@apple_support//:ios_sim_arm_platform=@apple_support//platforms:ios_sim_arm).

[keith]

sounds fine, i also think it would be fine if they were still in the platforms package, but no strong pref

[brentleyjones]

--@apple_support//platforms:ios_sim_arm_platform=//custom_platforms:ios_sim, yeah, that's not too bad, will go with that.