Don't treat as separate symbols if unversioned symbol is undefined.

Don't treat as separate symbols if unversioned symbol is undefined.

When we see an unversioned symbol reference in a shared library, followed
by a default definition of the symbol in another shared library, we were
treating them as separate symbols. That should only happen when both are
definitions.

gold/
	PR gold/20238
	* symtab.cc (Symbol_table::define_default_version): Check that
	unversioned symbol is defined.