* lib/dwarf.exp (Dwarf): New variable _abbrev_section.
	(_handle_DW_TAG): Use it.
	(cu, tu): Replace parameters is_64, version, addr_size with options.
	All callers updated.  Add Fission support.
	* gdb.dwarf2/implptrconst.exp: Update callers of "cu".
	* gdb.dwarf2/method-ptr.exp: Ditto.
	* gdb.dwarf2/nostaticblock.exp: Ditto.
	* gdb.dwarf2/subrange.exp: Ditto.
	* gdb.dwarf2/missing-sig-type.exp: Update callers of "cu", "tu".