adjust mach-o default GAS sections.

adjust mach-o default GAS sections.

gas:

	* as.c (perform_an_assembly_pass): Do not create text, data and bss
	sections for MACH-O.  Do not switch to the text section.
	* config/obj-macho.c (obj_mach_o_segT_from_bfd_name): Forward decl.
	(mach_o_begin): Startup with only text section unless suppressed.
	* config/obj-macho.h (obj_begin): define to mach_o_begin ().

gas/testsuite:

	* gas/mach-o/sections-1.d: Amend to recognize that bss is not emitted
	by default.
	* gas/mach-o/sections-2.d: New.