s390: Add get_lowcore() function

Add a get_lowcore() function which returns the address
of lowcore (currently always NULL). This function will
be used as a replacement of the S390_lowcore macro.
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/include/asm/lowcore.h

@@ -215,6 +215,11 @@ struct lowcore {
 
 #define S390_lowcore (*((struct lowcore *) 0))
 
+static __always_inline struct lowcore *get_lowcore(void)
+{
+	return NULL;
+}
+
 extern struct lowcore *lowcore_ptr[];
 
 static inline void set_prefix(__u32 address)