Fix bootstrap on Cygwin

On Cygwin I get this error, and I have to run configure.py manually:
```
$ ./bootstrap.py
Building ninja manually...
Building ninja using itself...
Usage: configure.py [options]

configure.py: error: option --platform: invalid choice: 'cygwin' (choose from 'linux', 'darwin', 'freebsd', 'openbsd', 'solaris', 'sunos5', 'mingw', 'msvc', 'gnukfreebsd8', 'bitrig')
```

This adds 'cygwin' to the list in `platform_helper` so bootstrapping works for native Cygwin builds.