The list of frozen modules in Python/frozen.c is generated by Tools/scripts/freeze_modules.py.  Currently we freeze both posixpath and ntpath, even though for startup we only need one of the two (depending on the OS).  In this case both modules are available on all platforms (and only os.path differs), so we might be okay to leave both frozen.  The cost isn't high.

However, we may need to accommodate freezing a module only on a subset of supported OSes:

* if the extra cost (to the size of the executable) is too high
* if there's an OS-specific module that is only available on that OS

In that case, we'd need to generate the appropriate ifdefs in Python/frozen.c.  We can't just exclude the module during generation since frozen.c is committed to the repo.