Cleanup: Remove 32bit unix-like os support

This PR aims remove all 32 bit support for Unix-like OS (in our case, Darwin86 and Linux32*) as Nupic don't support them longer. This is done in 2 commits:
- Remove all external libraries.
- Update pre-processor directives referencing them.

To reviewers:
The first commit just deletes `linux32` and `linux32arm` folders (click on "Browse Code" to check the results). The code changes appear only in the 2nd commit.