Fix processwrapper in multiplatform builds

[dzbarsky]

The problem is that cfg = "exec" is not well-defined when you have multiple exec platforms. We want to build the wrapper for the same platform that the toolchain will be used on, and the way to achieve that is by making it an attribute of the toolchain itself.

Previously, building on a linux remote executor from a windows host was broken.

[UebelAndre]

I haven't fully understood the implementation here but I feel like cfg = "exec" should be whatever the configuration is for the platform running an action. If this is not the case I feel this should be addressed in Bazel directly and not something rules have to account for. Is there an issue for this somewhere already?