S/390: Add support for z15 as CPU name.

S/390: Add support for z15 as CPU name.

So far z15 was identified as arch13. After the machine has been
announced we can now add the real name.

gas/ChangeLog:

2019-10-08  Andreas Krebbel  <krebbel@linux.ibm.com>

	* config/tc-s390.c (s390_parse_cpu): Add z15 as alternate CPU
	name.
	* doc/as.texi: Add z15 to CPU string list.
	* doc/c-s390.texi: Likewise.

opcodes/ChangeLog:

2019-10-08  Andreas Krebbel  <krebbel@linux.ibm.com>

	* s390-mkopc.c (main): Enable z15 as CPU string in the opcode
	table.