dwarf.exp: In 64-bit units, emit also abbrev offset as a 64-bit field

dwarf.exp: In 64-bit units, emit also abbrev offset as a 64-bit field

Dwarf::tu and Dwarf::cu allow selection of units with 64-bit offsets
through an option.  When selected, unit size is encoded properly, but
offset to abbreviation unit is still encoded in a 4-byte field.  This
patch fixes the problem.

Reproducer:

Dwarf::assemble "blah.s" {
    tu {is_64 1 version 4 addr_size 8} 0x1122334455667788 the_type {
	type_unit {} { the_type: }
    }

    cu {is_64 1 version 4 addr_size 8} {
	compile_unit {{language @DW_LANG_C}} {}
    }
}

gdb/testsuite:

	* lib/dwarf.exp  (Dwarf::cu, Dwarf::tu): Emit
	${_cu_offset_size} bytes abbrev offset.