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>

pc-bios/s390-ccw/start.S

@@ -10,10 +10,13 @@
  * directory.
  */
 
+#define STACK_SIZE        0x8000
+#define STACK_FRAME_SIZE  160
+
     .globl _start
 _start:
 
-    larl    %r15,stack + 0x8000     /* Set up stack */
+    larl    %r15,stack + STACK_SIZE - STACK_FRAME_SIZE   /* Set up stack */
 
     /* clear bss */
     larl    %r2,__bss_start