pc-bios/s390-ccw: Move the stack array into start.S

pc-bios/s390-ccw: Move the stack array into start.S

The stack array is only referenced from the start-up code (which is
shared between the s390-ccw.img and the s390-netboot.img), but it is
currently declared twice, once in main.c and once in netmain.c.
It makes more sense to declare this in start.S instead - which will
also be helpful in the next patch, since we need to mention the .bss
section in start.S in that patch.

While we're at it, let's also drop the huge alignment of the stack,
since there is no technical requirement for aligning it to page
boundaries.

Message-Id: <20230627074703.99608-4-thuth@redhat.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Eric Farman <farman@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>