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().