add dysymtab write support to bfd/mach-o.

add dysymtab write support to bfd/mach-o.

bfd:

	* mach-o.c (bfd_mach_o_write_symtab): Fill in the string table index
	as the value of an indirect symbol.  Keep the string table index in
	non-indirect syms for reference.
	(bfd_mach_o_write_dysymtab): New.
	(bfd_mach_o_primary_symbol_sort_key): New.
	(bfd_mach_o_cf_symbols): New.
	(bfd_mach_o_sort_symbol_table): New.
	(bfd_mach_o_mangle_symbols): Return early if no symbols.  Sort symbols.
	If we are emitting a dysymtab, process indirect symbols and count the
	number of each other kind.
	(bfd_mach_o_mangle_sections): New.
	(bfd_mach_o_write_contents): Split out some pre-requisite code into
	the command builder. Write dysymtab if the command is present.
	(bfd_mach_o_count_sections_for_seg): New.
	(bfd_mach_o_build_seg_command): New.
	(bfd_mach_o_build_dysymtab_command): New.
	(bfd_mach_o_build_commands): Reorganize to support the fact that some
	commands are optional and should not be emitted if there are no
	sections or symbols.
	(bfd_mach_o_set_section_contents): Amend comment.
	* mach-o.h: Amend and add to comments.
	(mach_o_data_struct): Add fields for dysymtab symbols counts and a
	pointer to the indirects, when present.
	(bfd_mach_o_should_emit_dysymtab): New macro.
	(IS_MACHO_INDIRECT): Likewise.

gas/testsuite:

	* gas/mach-o/dysymtab-1-64.d: New.
	* gas/mach-o/dysymtab-1.d: New.
	* gas/mach-o/symbols-1-64.d: New.
	* gas/mach-o/symbols-1.d: New.
	* gas/mach-o/symbols-base-64.s: New.
	* gas/mach-o/symbols-base.s: New.