xcoff buffer overflow

Much of the xcoff code is not well protected against fuzzed object file
attacks.  This sanity checks some values in ".loader".

	* xcofflink.c (xcoff_get_ldhdr): New function.
	(_bfd_xcoff_get_dynamic_symtab_upper_bound),
	(_bfd_xcoff_canonicalize_dynamic_symtab),
	(_bfd_xcoff_get_dynamic_reloc_upper_bound),
	(_bfd_xcoff_canonicalize_dynamic_reloc),
	(xcoff_link_add_dynamic_symbols),
	(xcoff_link_check_dynamic_ar_symbols): Use xcoff_get_ldhdr.