PE/COFF: use dyn_cast for the check of the target

The target may be a synthetic symbol like __ImageBase.  cast_or_null will ensure
that the atom is a DefinedAtom, which is not guaranteed, which was the original
reason for the cast_or_null.  Switch this to dyn_cast, which should enable
building of executables for WoA.  Unfortunately, the issue of missing base
relocations still needs to be investigated.

llvm-svn: 226246