S/390: Rename arch12 to z14

S/390: Rename arch12 to z14

This is a mechanical change not impacting code generation.  With that
patch I try to hide the artificial CPU name arch12 which we had to use
before the announcement of the IBM z14 machine.  arch12 of course
stays a valid option to -march and -mtune.  So this is just about
making the code somewhat easier to read.

gcc/ChangeLog:

2018-10-02  Andreas Krebbel  <krebbel@linux.ibm.com>

	* common/config/s390/s390-common.c: Rename PF_ARCH12 to PF_Z14.
	* config/s390/s390.h (enum processor_flags): Rename PF_ARCH12 to
	PF_Z14.  Rename TARGET_CPU_ARCH12 to TARGET_CPU_Z14,
	TARGET_CPU_ARCH12_P to TARGET_CPU_Z14_P, TARGET_ARCH12 to
	TARGET_Z14, and TARGET_ARCH12_P to TARGET_Z14_P.
	* config/s390/s390.md: Likewise. Rename also the cpu attribute
	value from arch12 to z14.

From-SVN: r264796