Add a generic .bss directive for ELF based targets.

	PR 3136
	* config/obj-elf.c (elf_pseudo_table): Add entry for .bss.
	(obj_elf_bss): New function.  Change to the .bss section.
	Support an optional subsection number.
	(obj_elf_change_section): Call obj_elf_section_change_hook.
	(obj_elf_section): Likewise.
	(obj_elf_data): Likewise.
	(obj_elf_text): Likewise.
	(obj_elf_struct): Likewise.
	(obj_elf_subsection): Likewise.
	(obj_elf_previous): Likewise.
	* config/obj-elf.h (obj_elf_bss): Prototype.
	* doc/as.texi (Bss): New node.