Keep natives source code in external strings instead of putting

Keep natives source code in external strings instead of putting
it in regular flat strings that are part of the snapshot.
After this change we don't need libraries-empty.cc any more.  In
this change libraries-empty.cc is just a the same as libraries.cc
and the scons build builds it but does not use it.  We can move
in stages to a situation where it is not generated at all for all
the build systems that we have.
Review URL: http://codereview.chromium.org/360050

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3238 ce2b1a6d-e550-0410-aec6-3dcde31c8c00