Previously we relied on undefined behavior (printf("%s", NULL)), now
we explicitly emit "<unknown>" when we hit an external reference with
an unknown name.

BUG=392068
LOG=y
R=dslomov@chromium.org

Review URL: https://codereview.chromium.org/379843002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22302 ce2b1a6d-e550-0410-aec6-3dcde31c8c00