* elfcode.h (elf_set_section_contents): Support calling the backend

	* elfcode.h (elf_set_section_contents): Support calling the backend
	function elf_backend_begin_write_processing when just beginning to
	write an object file.

	* libelf.h (elf_backend_begin_write_processing): Declare.

	* elf{32,64}-target.h (elf_backend_begin_write_processing): Provide
	a default definition.
	(elf{32,64}_bed): Add elf_backend_begin_write_processing.

	* elf32-hppa.h (elf_hppa_tc_symbol): Delete extern declaration.
	(elf_hppa_tc_make_sections): Likewise.

	* elf32-hppa.c (symext_chain_built): Delete.
	(symext_chain_size): Renamed from symextn_contents_real_size.
	(elf32_hppa_backend_{begin,final}_write_processing): New functions.
	(add_entry_to_symext_chain): New function.
	(hppa_elf_set_section_contents): Ignore writes to the symbol extension
	section until it's been rebuilt internally.
	(hppa_elf_get_section_contents): Symbol extension section is no
	longer special.
	(elf_backend_{begin,final}_write_processing): Define.
	(elf_hppa_tc_make_sections): Simplify now that much code has
	migrated into elf32_hppa_backend_{being,final}_write_processing.