Remove current-back-end-name.

Also, remove a usage of target-platform-name and get the platform
from the current project instead.
Rename target-platform-name to default-platform-name.

This method gets data from the environment, we want to be able to
get it from the current project.
Add a target-platform-name that pulls from current-library-description().
Use target-platform-name rather than separate os/architecture comparisons.
Remove platform-namestring-info when selecting compiler-back-end.
Use target-platform-name rather than target-os-name in HARP.
Remove target-architecture-name().

Replaced one remaining usage with an explicit list of 64 bit targets.
Remove invalid import (binding doesn't exist) of default-platform-info.
[env] Fix comment.
find-registries now takes platform-name as symbol.
[cbe] Combine methods by using if rather than singleton dispatch.
[cbe] Use target-platform-name rather than target-os-name.
[dfmc] Remove unused target-os-name().
[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.