Fix for bug #23256 "doesn't build on alpha"

- compilation on the Alpha platform was broken because the Alpha-specific code was not updated after replacing the SIGRETURN_FRAME_COUNT constant with a variable

sql/stacktrace.c

@@ -186,7 +186,7 @@ terribly wrong...\n");
 
 #if defined(__alpha__) && defined(__GNUC__)
     uchar** new_fp = find_prev_fp(pc, fp);
-    if (frame_count == SIGRETURN_FRAME_COUNT - 1)
+    if (frame_count == sigreturn_frame_count - 1)
     {
       new_fp += 90;
     }