swift234

Refactored built-script-impl:
- Use consistent terminology for target platforms, disambiguate uses of
  'deployment_target' in to 'host' and 'stdlib_targets'
- Changed parameters to build script:
  - install-prefix no longer has to include toolchain-prefix on OSX
  - swift-sdks has been replaced by stdlib-deployment-targets
  - built-stdlib-deployment-targets lets you restrict which of the
    stdlib-deployment targets are built
- Calculate stdlib_targets per host, to facilitate future cross-compiling
- Only use executables from LOCAL_HOST
- Only execute testable targets of the LOCAL_HOST
- Remove some cases of switching on 'uname' to facilitate future
  cross-compiling