IBM Z: Define STACK_CHECK_MOVING_SP

With -fstack-check the stack probes emitted access memory below the
stack pointer.

gcc/ChangeLog:

	* config/s390/s390.h (STACK_CHECK_MOVING_SP): New macro
	definition.

gcc/config/s390/s390.h

@@ -332,6 +332,11 @@ extern const char *s390_host_detect_local_cpu (int argc, const char **argv);
 
 #define STACK_SIZE_MODE (Pmode)
 
+/* Make the stack pointer to be moved downwards while issuing stack probes with
+   -fstack-check.  We need this to prevent memory below the stack pointer from
+   being accessed.  */
+#define STACK_CHECK_MOVING_SP 1
+
 #ifndef IN_LIBGCC2
 
 /* Width of a word, in units (bytes).  */