s390: add z14 cpu model

- add a CPU model for the IBM z14 which was announced on July 17th 2017
- update linux headers to 4.13-rc0 to get a fix for an ioctl definition

# gpg: Signature made Tue 18 Jul 2017 09:56:24 BST
# gpg:                using RSA key 0x117BBC80B5A61C7C
# gpg: Good signature from "Christian Borntraeger (IBM) <borntraeger@de.ibm.com>"
# Primary key fingerprint: F922 9381 A334 08F9 DBAB  FBCA 117B BC80 B5A6 1C7C

* remotes/borntraeger/tags/s390x-20170718:
  s390x/cpumodel: z14 cpu models
  linux header sync against v4.13-rc1

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>