Cleanup RelocInfo::NONE usage.

There are now NONE and NONE64 RelocInfo types, but only ARM uses them
both at the same time. They were added in:
  https://chromiumcodereview.appspot.com/11191029/

I'll rename NONE to NONE32 in a later CL.

This CL cleans up the RelocInfo::NONE usage by:
 - Using RelocInfo::IsNone when testing for NONE-ness.
 - Using NONE on 32-bit platforms (MIPS and IA32), and NONE64 on 64-bit
   platforms (x64).

This cleans up the code and prevents it from evolving bugs in the future
because NONE32 and NONE64 are used in misleading ways.

R= ulan@chromium.org

Review URL: https://chromiumcodereview.appspot.com/11695006
Patch from JF Bastien <jfb@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13307 ce2b1a6d-e550-0410-aec6-3dcde31c8c00