Fix issues with gold undefined symbol diagnostics.

Fix issues with gold undefined symbol diagnostics.

PR binutils/15435 complains that gold issues a visibility error for an
weak undefined symbol with hidden visibility. The message should be
suppressed if the symbol is a weak undef.

An earlier patch to add an extra note about key functions when a class's
vtable symbol is undefined missed a case where the reference to the
vtable came from a shared library. This patch moves the check to a
lower-level routine that catches both cases.

gold/

2014-02-05  Cary Coutant  <ccoutant@google.com>

	* errors.cc (Errors::undefined_symbol): Move undef vtable symbol
	check to here.
	* target-reloc.h (is_strong_undefined): New function.
	(relocate_section): Move undef vtable symbol check from here.
	Check for is_strong_undefined.