Introduce two separate classes of processor detection:

Introduce two separate classes of processor detection:
- TARGET, the architecture we will generate code for.
  This is brought it from the build system.
- HOST, the architecture our C++ compiler is building for.
  This is detected automatically based on compiler defines.

This adds macros for 32 or 64 bit, and cleans up some
include conditionals, etc.

Review URL: http://codereview.chromium.org/99355

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1864 ce2b1a6d-e550-0410-aec6-3dcde31c8c00