Sanity check elf_sym_hashes indexing

Sanity check elf_sym_hashes indexing

I'm a little surprised we haven't already had fuzzing reports of
indexing off the end of sym_hashes.  The idea here is to preempt such
bugs.  One wrinkle is that ppc64 can't leave a zero symtab_hdr when
setting up sym_hashes for the fake stub bfd.

	* elf-bfd.h (struct elf_reloc_cookie): Add "num_sym".
	(_bfd_elf_get_link_hash_entry): Update declaration.
	* elf-eh-frame.c (find_merged_cie): Sanity check reloc symbol
	index.
	* elf64-ppc.c (use_global_in_relocs): Fake up symtab_hdr for
	stub bfd.
	* elflink.c (_bfd_elf_get_link_hash_entry): Add "num_sym"
	param.  Check symndx against it.  Update all calls.
	(set_symbol_value): Add "num_sym" param and update all calls.
	(elf_link_input_bfd): Add "num_syms" var and use for above.
	(init_reloc_cookie): Set "cookie->num_syms".
	* elf64-x86-64.c (elf_x86_64_scan_relocs): Pass symtab number
	of entries to _bfd_elf_get_link_hash_entry.
	* elfxx-x86.c (_bfd_x86_elf_check_relocs): Likewise.
	(_bfd_x86_elf_link_relax_section): Likewise.