add TRAP_UNK si_code to signal.h from linux v4.18

used for undiagnosed trap exceptions where linux previously set si_code
to 0. new in linux commit db78e6a0a6f9f7d7277965600eeb1a5b3a6f55a8

include/signal.h

@@ -233,6 +233,7 @@ void (*sigset(int, void (*)(int)))(int);
 #define TRAP_TRACE 2
 #define TRAP_BRANCH 3
 #define TRAP_HWBKPT 4
+#define TRAP_UNK 5
 #define POLL_IN 1
 #define POLL_OUT 2
 #define POLL_MSG 3