--as-needed change wrt undefined weak symbols

[Alan Modra]

http://sourceware.org/ml/binutils/2013-01/msg00188.html

bfd/
	PR ld/12549
	elflink.c (elf_link_add_object_symbols): Exclude weak refs when
	considering whether an --as-needed library is needed.
ld/
	* ld.texinfo (--as-needed): Update.
ld/testsuite/
	* ld-elf/pr14862.out: Expect no output.

I'm committing the above patch.  Apologies to the bug reporter that
it's taken so long.

-- 
Alan Modra
Australia Development Lab, IBM