s390: align struct lowcore to double page size

Aligning struct lowcore to double page size allows to get rid of this
gcc warning:

In file included from ./arch/s390/include/asm/setup.h:56,
                 from ./arch/s390/include/asm/page.h:36,
                 from ./arch/s390/include/asm/user.h:11,
                 from ./include/linux/user.h:1,
                 from ./include/linux/elfcore.h:5,
                 from ./include/linux/crash_core.h:6,
                 from ./include/linux/kexec.h:18,
                 from arch/s390/purgatory/purgatory.c:10:
./arch/s390/include/asm/lowcore.h:189:1: warning: alignment 1 of 'struct
lowcore' is less than 8 [-Wpacked-not-aligned]
 } __packed;
Acked-by: NChristian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: NVasily Gorbik <gor@linux.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/include/asm/lowcore.h

@@ -185,7 +185,7 @@ struct lowcore {
 	/* Transaction abort diagnostic block */
 	__u8	pgm_tdb[256];			/* 0x1800 */
 	__u8	pad_0x1900[0x2000-0x1900];	/* 0x1900 */
-} __packed;
+} __packed __aligned(8192);
 
 #define S390_lowcore (*((struct lowcore *) 0))