kboot: update copyright on these files.

host_syscalls.c: I've written, so put Netflix copyright on. It's
possible in the confusion that Nathan wrote the host_gettimeofday
implementation.

syscall_nr: These files likely can't enjoy copyright protection since
they are just facts (the per-arch Linux system calls), so add a note
they are in the public domain.

Sponsored by:		Netflix

(cherry picked from commit 8f7327dceecc225029b17378e4a26ae0b73a0a49)

stand/kboot/include/arch/aarch64/syscall_nr.h

@@ -1,3 +1,7 @@
+/*
+ * This file is in the public domain.  It only recounts facts in the only way
+ * possible.
+ */
 #define SYS_close		 57
 #define	SYS_dup			 23
 #define SYS_exit		 93

stand/kboot/include/arch/amd64/syscall_nr.h

@@ -1,3 +1,7 @@
+/*
+ * This file is in the public domain.  It only recounts facts in the only way
+ * possible.
+ */
 #define SYS_close		  3
 #define	SYS_dup			 32
 #define	SYS_exit		 60

stand/kboot/include/arch/powerpc64/syscall_nr.h

@@ -1,3 +1,7 @@
+/*
+ * This file is in the public domain.  It only recounts facts in the only way
+ * possible.
+ */
 #define SYS_close		  6
 #define	SYS_dup			 41
 #define SYS_exit		  1

stand/kboot/libkboot/host_syscalls.c

@@ -1,3 +1,9 @@
+/*
+ * Copyright (c) 2022-2024, Netflix, Inc.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
 #include "host_syscall.h"
 #include "syscall_nr.h"
 #include <stand.h>