It has certain overheads even when gdb is not attached so it is guarded by ENABLE_GDBJIT_INTERFACE define and --gdbjit flag.

Review URL: http://codereview.chromium.org/5965011

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6367 ce2b1a6d-e550-0410-aec6-3dcde31c8c00