Hi everyone!

Thanks to Léo's help, as of commit
8159ce1970d91567468cf1bacac313099a009d2a, the master branch now contains
all the changes necessary to cross-compile powerpc64-linux bootstrap
binaries.  I've done this without substitutes by running the following
commands on an x86_64-liinux machine.

First, to ensure you're using commit
8159ce1970d91567468cf1bacac313099a009d2a, put something like this in
your ~/.config/guix/channels.scm file:

(list (channel
        (name 'guix)
        (url "https://git.savannah.gnu.org/git/guix.git")
        (commit
          "8159ce1970d91567468cf1bacac313099a009d2a")))

Then, run these commands:

# Confirm you're using the right guix.
guix describe
# Clear as many GC roots as possible, and do a GC run.
guix gc --delete-generations
# Build the bootstrap tarballs without substitutes.
guix build --no-substitutes --target=powerpc64-linux-gnu bootstrap-tarballs

After a few hours, you should see the following message:

successfully built /gnu/store/icnj0m294b94pc3rhpmkz6zc41w8vyqj-bootstrap-tarballs-0.drv
/gnu/store/dy4fhy2vqyxqfad59mfdqdd0qpcrwmyx-bootstrap-tarballs-0

On my end, the SHA-512 hashes of the binaries are:

--8<---------------cut here---------------start------------->8---
426e5f1d0d7023a90e73286ccda1fa55a359301e998a19dfe00f5b4f5d387e69d7a247f47056f41e609393893b0238a908698fbd28d73b183b32a5dadcfe9fbb  binutils-static-stripped-2.34-powerpc64-linux-gnu.tar.xz
8aca7f332a1ba8e3c2225c161a7545b0a04ddd690d164dc97afee9c9ea067b0c49bc155e9f06d285c22e24cdd16d91e59730af5f1dd9efcda13a26bede5948a2  gcc-stripped-5.5.0-powerpc64-linux-gnu.tar.xz
a717a420e765accf12cfc1e18ebed76e9359ee58e8781601ca9066ced59196f88a528ddc554c0f57c77e2c01908cafe596f3c8d1df135beb4cae4073b9a999d2  glibc-stripped-2.31-powerpc64-linux-gnu.tar.xz
e2e70c7fcc477fced12eb76704212f9bda0e1ec2cf40137ff6a32a85ca75fec10ec20076b73698438e48c3ce45d24542aa309bb99274f4c3d4f9d49ec9d1dd7b  guile-static-stripped-2.0.14-powerpc64-linux-gnu.tar.xz
04d9203467ecb48e9f1fca5130199c292212d4d119153778d398899aeef517fc8bce5d25f3505063f38e433fa09e3c723a6da5dee4943dbc9d3728279356879b  static-binaries-0-powerpc64-linux-gnu.tar.xz
--8<---------------cut here---------------end--------------->8---

Hopefully, you'll get identical results!  You don't have to run "guix
gc" if you don't want to, but doing so will increase the likelihood of
catching nondeterminism issues propagated from dependencies (which seem
unlikely, but you never know).  It took 3 or 4 for me hours on a modern
16-core machine.

Once we verify the binaries, we can actually start using them to build
stuff!  Léo has already gotten an optimistic start on that work, and
many things are building successfully.  Exciting!!

-- 
Chris