Don't override definition a shared object by one in a later shared object.

Don't override definition a shared object by one in a later shared object.

In PR 16979, a reference to malloc is being resolved to an unversioned
reference in libmalloc.so. When linked with --as-needed, however, the
dynamic table does not list libmalloc.so as a DT_NEEDED library.

If we have a reference to an unversioned symbol in a shared object,
and we later see a versioned definition in another shared object, we
were overriding the first definition with the second in the process of
defining the default version. As a result, we no longer think that the
first shared object was actually needed to resolve any symbols, and we
don't list it as a DT_NEEDED library.

This patch fixes the problem by treating the two definitions as separate
symbols, so the second definition does not override the first.

2016-03-30  Cary Coutant  <ccoutant@gmail.com>

gold/
	PR gold/16979
	* symtab.cc (Symbol_table::define_default_version): Check for case
	where symbols are both in different shared objects.