x32: Add <fixup-asm-unistd.h> and regenerate arch-syscall.h

X32 uses the same 64-bit syscall interface for set_thread_area.  But
__NR_set_thread_area is missing from <asm/unistd_x32.h>.  A kernel patch
was submitted:

From 7b05d5b43ae2545e0d4a3edb24205d18bc883626 Mon Sep 17 00:00:00 2001
From: "H.J. Lu" <hjl.tools@gmail.com>
Date: Sat, 15 Aug 2020 10:34:00 -0700
Subject: [PATCH] x86-64: Enable x32 set_thread_area

X32 uses the common 64-bit syscall interface for set_thread_area.  Add
<fixup-asm-unistd.h> to provide __NR_set_thread_area.

Co-authored-by: Florian Weimer <fweimer@redhat.com>