fix string table generation for XCOFF64 .debug section

fix string table generation for XCOFF64 .debug section

bfd	* hash.c (struct bfd_strtab_hash): Remove xcoff field.
	Add length_field_size field.
	(_bfd_stringtab_init): Change prototype.
	Adapt to new length_field_size.
	(_bfd_xcoff_stringtab_init): Likewise.
	(_bfd_stringtab_add): Likewise.
	(_bfd_stringtab_emit): Likewise.
	* libbfd-in.h (_bfd_xcoff_stringtab_init):
	Change prototype.
	* libbfd.h: Regenerate.
	* xcofflink.c (_bfd_xcoff_bfd_link_hash_table_create):
	Call _bfd_xcoff_stringtab_init with isxcoff64 value.