Default elf_backend_post_process_headers to set OSABI

Default elf_backend_post_process_headers to set OSABI

	* elf-bfd.h (_bfd_elf_set_osabi): Renamed to ...
	(_bfd_elf_post_process_headers): This.
	* elf.c (_bfd_elf_compute_section_file_positions): Always
	call elf_backend_post_process_headers.
	(_bfd_elf_set_osabi): Renamed to ...
	(_bfd_elf_post_process_headers): This.
	* elf32-hppa.c (elf_backend_post_process_headers): Removed.
	* elf32-i370.c (elf_backend_post_process_headers): Likewise.
	* elf32-msp430.c (elf_backend_post_process_headers): Likewise.
	* elf32-ppc.c (elf_backend_post_process_headers): Likewise.
	* elf32-sparc.c (elf_backend_post_process_headers): Likewise.
	* elf64-hppa.c (elf_backend_post_process_headers): Likewise.
	* elf64-ppc.c (elf_backend_post_process_headers): Likewise.
	* elf64-sparc.c (elf_backend_post_process_headers): Likewise.
	* elf64-x86-64.c (elf_backend_post_process_headers): Likewise.
	*  (elf_backend_post_process_headers): Likewise.
	*  (elf_backend_post_process_headers): Likewise.
	* elf32-i386.c (elf_backend_post_process_headers): Don't
	define as _bfd_elf_set_osabi.
	(elf_i386_fbsd_post_process_headers): Updated.
	* elf32-mips.c (elf_fbsd_post_process_headers): Removed.
	(elf_backend_post_process_headers): Likewise.
	* elf32-tic6x.c (elf32_tic6x_set_osabi): Removed.
	(elf_backend_post_process_headers): Likewise.
	* elf64-mips.c (elf_fbsd_post_process_headers): Removed.
	(elf_backend_post_process_headers): Likewise.
	* elfn32-mips.c (elf_fbsd_post_process_headers): Removed.
	(elf_backend_post_process_headers): Likewise.
	* elfnn-aarch64.c (elfNN_aarch64_post_process_headers): Updated.
	* elfxx-target.h (elf_backend_post_process_headers): Default
	to _bfd_elf_post_process_headers.