Minor refactoring for cross compilation code.

1. Instead of using cross*/tools* in the project names, we simply place the
   project.cmake files under cross or tools subdirs (similar to what we do for
   all OSes.
2. Added ability to specify "SITE" specific defaults using
   ParaViewDefaults.${site}.cmake file.

Change-Id: I58e3c4cf1b5a903665d97188afbeca19a77192e7