s390/entry: Add base register to CHECK_VMAP_STACK/CHECK_STACK macro

In preparation of having lowcore at different address than zero,
add the base register to CHECK_VMAP_STACK and CHECK_STACK. No
functional change, because %r0 is passed to the macro.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>