swift234

[build-script] There can only be one host target

`build-script-impl` currently maintains a list of
`NATIVE_TOOLS_DEPLOYMENT_TARGETS` -- host machine targets, for which
the resulting binaries can be run on the current machine.

However, there is only ever *one* host machine. This commit:

- Changes the `NATIVE_TOOLS_DEPLOYMENT_TARGETS` list parameter into a
  single string parameter, `HOST_TARGET`.
- Promotes the logic to detect the host target to Python, and places it
  in the `swift_build_support` module.
- Removes the hard-coded "macosx_x86_64" path to the LLVM and Clang
  TableGen -- thereby unblocking future work to make cross-compilation
  possible on platforms other than OS X.
- Also promotes cross-compilation target validation to Python, placing
  it in the `swift_build_support` module.