DO NOT USE THIS BUGGY SOLUTION! Refer to https://wiki.termux.com/wiki/PRoot

another PC run nginx Then we have mirror: 10.100.100.34/$arch/$repo

    server {
        listen       [::]:80;
        listen       80;
        #server_name  localhost;
    root /var/www/html;
        index  index.html index.htm;

        #charset koi8-r;

        #access_log  logs/host.access.log  main;

        location /aarch64 {
        proxy_pass https://mirror.tuna.tsinghua.edu.cn/archlinuxarm/aarch64;
        }
......

android chroot:

prepare root.

mount -t proc /proc proc/
mount -o bind /sys sys/
mount -o bind /dev dev/
# Maybe we should bind every mountpoint manually. Write a script, or copy from debian-arm blog. 
chroot /path/to/.../root/

inside: /bin/bash

export PATH=/usr/bin
export LD_LIBRARY_PATH=/usr/lib

# we should run `pacman-key --init` and `pacman-key --populate archlinuxarm`. But I failed. 
vi /etc/pacman.conf # Set Siglevel to Never

# I don't know why DNS is still not working. Modify mirrorlist to use our repo. 
rm /etc/resolv.conf
echo 10.100.100.1 > /etc/resolv.conf