Add option to disable loop vectorization on 32bit win32 (workaround for #649)

- Add environment variable to turn loop vectorization on/off.
   - Off by default on 32-bit win32 
`platform.architecture` is not reliable (see note in https://docs.python.org/3.3/library/platform.html#platform.architecture).
Ilan suggested this and has been my favorite way to query for bitness.  I know it works, but I don't remember the details.