The file aarch64chroot.tar.xz is a ready made Pi3 64 bit chroot.
It uses qemu to emulate an arm64 CPU.

The tarball unpacks inside of 5G. 
To emerge things you need to provide the missing bits. 

As provided, the chroot contains a statically linked qemu
to suit any 64 bit CPU.

The chroot started life as a 
crossdev -t aarch64-unknown-linux-gnu
system. It can still be used that way. 
make.conf is a symlink to one of two real make.conf files.
One for cross compiling, one for chrooting. 

There is a script to set up the chroot, prep_chroot, that
mouts all the missing bits from the host to make the chroot work.
You will need to edit that so it works with your disk layout.
When its all in place

chroot /usr/aarch64-unknown-linux-gnu /bin/bash --login 

will get you into the emulated aarch chroot.
Don't forget to set the environment once inside.

Take care how and where you unpack the tarball. 
Untarring arm64 files over your amd64 install will ruin you day.

/lib/fimware has been --excluded from the tarball along with
/usr/src and some other junk. The script that made the tarball
is included.

There is some useful information on the Gentoo wiki.
https://wiki.gentoo.org/wiki/User:NeddySeagoon/Pi3_Build_Root
https://wiki.gentoo.org/wiki/Embedded_Handbook/General/Compiling_with_qemu_user_chroot

This tarball is not a ready made Pi3 install. e.g. /boot is empty.
There are no passwords in /etc/shadow and so on. 

Users with nVidia graphics hardware, using the nouveau driver
can run Xfce4 from inside the chroot. Its not fast but it starts.