add .zerofill to mach-o GAS.

add .zerofill to mach-o GAS.

gas:

	* config/obj-macho.c (obj_mach_o_segT_from_bfd_name): Tidy definition.
	(obj_mach_o_get_section_names): New (split from obj_mach_o_section).
	(obj_mach_o_make_or_get_sect): Likewise.
	(obj_mach_o_section): Split out the functionality shared with zerofill.
	(obj_mach_o_zerofill): New.
	(obj_mach_o_common_parse): Ensure whitespace is skipped.
	(mach_o_pseudo_table): Add .zerofill.

gas/testsuite:

	* gas/mach-o/zerofill-1.d: New.
	* gas/mach-o/zerofill-1.s: New.