Kernel: Fill some siginfo and ucontext fields on SA_SIGINFO

There's no reason to fill in any of these fields if SA_SIGINFO is not
given, as the signal handler won't be reading from them at all.

Kernel/API/POSIX/ucontext.h

@@ -54,6 +54,7 @@ typedef struct __ucontext {
 #define SI_TIMER 0x40000002
 #define SI_ASYNCIO 0x40000003
 #define SI_MESGQ 0x40000004
+#define SI_NOINFO 0x40000042
 
 #ifdef __cplusplus
 }