Platform wasn't being set in the right location.

This was causing fontconfig.cmake missing errors for HOST builds on Linuxes.
Fixed that.

Change-Id: I40c81d12b3c9867fba059c57bffc80c1457e032b
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