pc-bios/s390-ccw: Provide space for initial stack frame in start.S

pc-bios/s390-ccw: Provide space for initial stack frame in start.S

Providing the space of a stack frame is the duty of the caller,
so we should reserve 160 bytes before jumping into the main function.
Otherwise the main() function might write past the stack array.

While we're at it, add a proper STACK_SIZE macro for the stack size
instead of using magic numbers (this is also required for the following
patch).

Reviewed-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Message-Id: <20230627074703.99608-3-thuth@redhat.com>
Reviewed-by: Eric Farman <farman@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>