genlink tidy

Some of the declarations in genlink.h are not used in current sources
apart from needing them in linker.c, so delete and/or move them there.
The patch also fixes a FIXME.  It's actually quite easy to return
a failure from a hash traversal function.

	* genlink.h (_bfd_generic_link_hash_newfunc): Delete.
	(_bfd_generic_link_output_symbols),
	(generic_write_global_symbol_info),
	(_bfd_generic_link_write_global_symbol): Move to..
	* linker.c: ..here, making functions static.
	(generic_write_global_symbol_info): Add "failed".
	(_bfd_generic_final_link): Handle wginfo.failed.
	(_bfd_generic_link_write_global_symbol): Set wginfo->failed
	on memory failures and return false rather than aborting.