Differentate between code target pointers and heap pointers in constant pools.

Separate out code target pointers from normal heap pointer entries in constant
pool arrays so that the GC can correctly relocate these pointers using the
appropriate mechanism.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/183883011

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19773 ce2b1a6d-e550-0410-aec6-3dcde31c8c00