s390: Add support for z17 as CPU name

s390: Add support for z17 as CPU name

So far IBM z17 was identified as arch15.  Add the real name, as it has
been announced. [1]

[1]: IBM z17 announcement letter, AD25-0015,
     https://www.ibm.com/docs/en/announcements/z17-makes-more-possible

gas/
	* config/tc-s390.c (s390_parse_cpu): Add z17 as alternate CPU
	name for arch15.
	* doc/c-s390.texi: Likewise.
	* doc/as.texi: Likewise.

opcodes/
	* s390-mkopc.c (main): Add z17 as alternate CPU name for arch15.

Signed-off-by: Jens Remus <jremus@linux.ibm.com>