[build] Add default-platform-name-setter.
[build,projects] Thread target platform info through more.
[env] Forward platform-name / platform-name-setter from env to underlying projects.
[cbe] Add comment saying what version of Open Dylan generated the code and for what platform.
[build] Reset build-script when setting platform-name.
[env,console] Allow changing target platform at run-time.

This adds a 'target-platform' property when running dylan-compiler
and related tools. Setting this will reset the build script to the
default for the new platform name as well.
[projects] Pass platform-name key through to initializer.