Add x64 targets to the v8 Xcode project. 
Move platform specific files into their own groups. 

BUG= 
TEST=

Code review URL: http://codereview.chromium.org/5958020/

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6193 ce2b1a6d-e550-0410-aec6-3dcde31c8c00