Move NumberOfProcessorsOnline from CPU to OS

It's really more an OS-level information, and this way the default
platform doesn't depend on CPU-level details

BUG=none
R=yangguo@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/300713002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21501 ce2b1a6d-e550-0410-aec6-3dcde31c8c00