runtime: fix SI_USER for FreeBSD

Ref: http://svnweb.freebsd.org/base/head/sys/sys/signal.h?revision=HEAD&view=markup

R=golang-dev, devon.odell, r
CC=golang-dev
https://golang.org/cl/6490118

src/pkg/runtime/os_freebsd.h

@@ -15,7 +15,7 @@ int32	runtime·sysctl(uint32*, uint32, byte*, uintptr*, byte*, uintptr);
 void	runtime·raisesigpipe(void);
 
 #define	NSIG 33
-#define	SI_USER	0
+#define	SI_USER	0x10001
 
 #define RLIMIT_AS 10
 typedef struct Rlimit Rlimit;