kboot: Add host_exit and use it to implement exit()

Clients of libsa are expected to implement exit(). The current exit just
loops forever. It is better to really exit: when running as init that
will reboot the system. When not running as init, other programs can
recover (not that we support running as init, but when we do in the
future, this is still the rigtht thing).

Sponsored by: Netflix

stand/kboot/arch/aarch64/syscall_nr.h

@@ -1,5 +1,6 @@
 #define SYS_close		 57
 #define	SYS_dup			 23
+#define SYS_exit		 93
 #define SYS_fstat		 80
 #define SYS_getdents64		 61
 #define	SYS_getpid		172

stand/kboot/arch/amd64/syscall_nr.h

@@ -1,5 +1,6 @@
 #define SYS_close		  3
 #define	SYS_dup			 32
+#define	SYS_exit		 60
 #define SYS_getdents64		217
 #define	SYS_getpid		 39
 #define SYS_gettimeofday	 96

stand/kboot/arch/powerpc64/syscall_nr.h

@@ -1,5 +1,6 @@
 #define SYS_close		  6
 #define	SYS_dup			 41
+#define SYS_exit		  1
 #define SYS_fstat		108
 #define SYS_getdents64		202
 #define	SYS_getpid		 20

stand/kboot/host_syscall.h

@@ -155,6 +155,7 @@ struct host_dirent64 {
  */
 int host_close(int fd);
 int host_dup(int fd);
+int host_exit(int code);
 int host_fstat(int fd, struct host_kstat *sb);
 int host_getdents64(int fd, void *dirp, int count);
 int host_getpid(void);

stand/kboot/host_syscalls.c

@@ -19,6 +19,12 @@ host_dup(int fd)
 	return host_syscall(SYS_dup, fd);
 }
 
+int
+host_exit(int code)
+{
+	return host_syscall(SYS_exit, code);
+}
+
 /* Same system call with different names on different Linux architectures due to history */
 int
 host_fstat(int fd, struct host_kstat *sb)

stand/kboot/main.c

@@ -303,7 +303,7 @@ main(int argc, const char **argv)
 void
 exit(int code)
 {
-	while (1); /* XXX: host_exit */
+	host_exit(code);
 	__unreachable();
 }