Sort the syscalls.mk files, give all generated files the same header.

No non-comment changes to the .S files.

Change-Id: Iafcfd004c3ea92b64268f80ab16df615b97cefac
diff --git a/libc/arch-mips/syscalls/removexattr.S b/libc/arch-mips/syscalls/removexattr.S
index 55c8e59..29df5d2 100644
--- a/libc/arch-mips/syscalls/removexattr.S
+++ b/libc/arch-mips/syscalls/removexattr.S
@@ -1,4 +1,5 @@
-/* autogenerated by gensyscalls.py */
+/* Generated by gensyscalls.py. Do not edit. */
+
 #include <asm/unistd.h>
     .text
     .globl removexattr