gas: Fix .debug_info CU header for --gdwarf-5

gas: Fix .debug_info CU header for --gdwarf-5

DWARF5 CU headers have a new unit type field and move the abbrev offset
to the end of the header.

gas/ChangeLog:

	* dwarf2dbg.c (out_debug_info): Emit unit type and abbrev offset
	for DWARF5.
	* gas/testsuite/gas/elf/dwarf-4-cu.d: New file.
	* gas/testsuite/gas/elf/dwarf-4-cu.s: Likewise.
	* gas/testsuite/gas/elf/dwarf-5-cu.d: Likewise.
	* gas/testsuite/gas/elf/dwarf-5-cu.s: Likewise.
	* testsuite/gas/elf/elf.exp: Run dwarf-4-cu and dwarf-5-cu.