unix: update generation scripts for arm64/ppc64

This applies the following CLs from the syscall package:
http://golang.org/cl/9962
http://golang.org/cl/9966
http://golang.org/cl/9924

It also adds arm64/ppc64 support to mkall.sh and mksysnum_linux.pl.

Change-Id: I84fbfde2a2876dbf6d16c6159e847195c274b396
Reviewed-on: https://go-review.googlesource.com/10037
Reviewed-by: Rob Pike <r@golang.org>

unix/mkerrors.sh

@@ -88,7 +88,9 @@ includes_FreeBSD='
 includes_Linux='
 #define _LARGEFILE_SOURCE
 #define _LARGEFILE64_SOURCE
+#ifndef __LP64__
 #define _FILE_OFFSET_BITS 64
+#endif
 #define _GNU_SOURCE
 
 #include <bits/sockaddr.h>