Provide way to disable transition to default platform

`rules_apple` has a default transition to a mac osx platform. In our case, I believe it is transitioning to the same platform as our normal OSX build, but this is a pain as we want to skip targets on other platforms if they aren't compatible.

It seems that transitioning makes `target_compatible_with` take no effect, the target doesn't get filtered out, and then bazel raises a confusing error (in this case about there not being a toolchain to compile for OSX with host platform linux-x64), rather than skipping the target. I want the target to be filtered out by bazel's normal handling of targets incompatible with the target platform, to make things like `bazel build --nobuild //...` and `bazel test //...` work on every platform target for a multi-architecture codebase that has some parts that are OSX-specific.

Start by tracing the rules_apple default transition and how Bazel evaluates target_compatible_with for Apple targets. Reproduce the behavior with `bazel build --nobuild //...` and `bazel test //...` on a non-macOS host; done means users can disable the transition and incompatible targets are filtered instead of producing a toolchain error.

Written by the indexing model from the issue text.