MAINT-7514: Add (32) or (64) to registry for "install again?" prompt.

Specifically, append (32) or (64) to the four-part version number stored in
the registry entry used to detect whether this viewer has already been
installed. This is injected as a new VERSION_REGISTRY NSIS variable.

(It was tempting to simply change the value of VERSION_LONG with the embedded
address size. However, there is one other use of VERSION_LONG in the NSIS
template. That use is the subject of MAINT-7533.)

Synthesize the VERSION_REGISTRY value in viewer_manifest.py and add it to the
substitution dict used to populate the NSIS template.

ADDRESS_SIZE isn't passed into viewer_manifest.py, but it can be inferred from
the existing 'arch' parameter: 'arch' as well as 'platform' is used to select
the specific subclass of the ViewerManifest class to instantiate for this run.
Add an appropriate address_size attribute to every such subclass.

Change a couple existing tests on 'arch' to tests on self.address_size instead
-- clearer to the maintainer.

Also, given that subclass selection mechanism, the ViewerManifest base class
shouldn't need if / elif tests on 'platform'. Make build_data_json_platform a
class attribute as well, removing the base-class stanza that dynamically
examines 'platform' and 'arch'.

Similarly, move platform-specific tweaks to the build_data_dict used to
populate build_data.json into a new finish_build_data_dict() method overridden
by individual platform subclasses.

Encapsulate the logic around running the Windows code-signing tool into a
sign() method, and call it as needed. For obtaining environment variables with
fallback values, use os.environ.get() instead of os.path.expandvars() with
tests on the returned value.