S/390: Add alternate processor names.

This patch adds alternate CPU names which adhere to the number of the
architecture document.  So instead of having z196, zEC12, and z13 you
can use arch9, arch10, and arch11.  The old cpu names stay valid and
should primarily be used.

The alternate names are supposed to improve compatibility with the IBM
XL compiler toolchain which uses the arch numbering.

opcodes/ChangeLog:

2016-09-12  Andreas Krebbel  <krebbel@linux.vnet.ibm.com>

	* s390-mkopc.c (main): Support alternate arch strings.

gas/ChangeLog:

2016-09-12  Andreas Krebbel  <krebbel@linux.vnet.ibm.com>

	* config/tc-s390.c (s390_parse_cpu): Support alternate arch
	strings.
	* doc/as.texinfo: Document new arch strings.
	* doc/c-s390.texi: Likewise.