Treat entry point symbol as an undefined atom.

With this patch the entry symbol is treated as an undefined symbol, to force
the resolver to resolve the entry symbol.

Differential Revision: http://llvm-reviews.chandlerc.com/D1524

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@189307 91177308-0d34-0410-b5e6-96231b3b80d8