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>