bfd_get_arch_size: return size from arch info on non-ELF targets.

bfd/
	* bfd.c (bfd_get_arch_size): Default is taken from arch.
mach-o: output output_section target_index to write relocs.

bfd/
	* mach-o-i386.c (bfd_mach_o_i386_swap_reloc_out): Use target index
	of output_section.
	* mach-o-x86-64.c (bfd_mach_o_x86_64_swap_reloc_out): Ditto.
mach-o: fix section name conversion from bfd to mach-o.

bfd/
	* mach-o.c (bfd_mach_o_convert_section_name_to_mach_o): Fix
	thinko on names length.
mach-o: reject 64 bit targets when not configured for.

bfd/
	* mach-o.c (bfd_mach_o_header_p): Reject 64 bit target when not
	configured for.