Add S390 support for linux-kernel target

Add S390 support for linux-kernel target

After implementing the new linux-kernel target and preparing s390-tdep.
It is now time to get everything to work.  Thus implement the hooks
required by the linux-kernel target and enable s390's privileged
registers.

gdb/ChangeLog:

	* s390-lk-tdep.h: New file.
	* s390-lk-tdep.c: New file.
	* Makefile.in (ALL_TARGET_OBS): Add s390-lk-tdep.o.
	(ALLDEPFILES): Add s390-lk-tdep.c.
	* s390-tdep.h: Define macros for address translation.
	* s390-tdep.c (s390-lk-tdep.h): New include.
	(s390_iterate_over_regset_sections): Enable privileged registers.
	(s390_core_read_description): Enable privileged registers.
	(s390_gdbarch_init): : Enable privileged registers and adjust.