pc-bios/s390-ccw: Use proper register names for Clang

pc-bios/s390-ccw: Use proper register names for Clang

When compiling the s390-ccw firmware with Clang 7.0.1, I get the
following errors:

pc-bios/s390-ccw/start.S:62:19: error: invalid use of length addressing
        stctg 0,0,0(15)
                  ^
pc-bios/s390-ccw/start.S:63:12: error: invalid use of length addressing
        oi 6(15), 0x2
           ^
pc-bios/s390-ccw/start.S:64:19: error: invalid use of length addressing
        lctlg 0,0,0(15)
                  ^
pc-bios/s390-ccw/start.S:76:19: error: invalid use of length addressing
        stctg 0,0,0(15)
                  ^
pc-bios/s390-ccw/start.S:77:12: error: invalid use of length addressing
        ni 6(15), 0xfd
           ^
pc-bios/s390-ccw/start.S:78:19: error: invalid use of length addressing
        lctlg 0,0,0(15)
                  ^
pc-bios/s390-ccw/start.S:79:12: error: invalid operand for instruction
        br 14
           ^

Let's use proper register names like in the rest of this file to fix it.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1547123559-30476-1-git-send-email-thuth@redhat.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>