Gah!
Dup2 is not implemented in go on aarch64, apparently intentionally.
I changed in file keybase/client/go/logger/redirect_stderr_nix.go this line
$this->bbcode_second_pass_code('', ' return syscall.Dup2(int(f.Fd()), 2)')
to
$this->bbcode_second_pass_code('', ' return syscall.Dup3(int(f.Fd()), 2, 0)')
(sorry, I don't know how to make patches)
But it fails to build for some other reason that I don't understand (I was using
this PKGBUILD, just changed arch to be aarch64).