gnu: bootstrap: Add support for the Hurd.

gnu: bootstrap: Add support for the Hurd.

On 3342a1182b15ec031f0ec6f602fd96c1dca3d4b0
   gnu: make-bootstrap: Use _IOLBF on Guile 2.0 only.

Run
    ./pre-inst-env guix build --target=i586-pc-gnu bootstrap-tarballs --verbosity=1

Producing

    /gnu/store/lhca65c997pvic5cfrpm0dasniwqlg2a-bootstrap-tarballs-0

With guix hash -rx /gnu/store/lhca65c997pvic5cfrpm0dasniwqlg2a-bootstrap-tarballs-0

    07jnq2by98f2a45k8wd2gj62iazvwfa4z7p3w3id4m1g0fdsvc3b

* gnu/packages/bootstrap.scm (%bootstrap-executables): Add entries for the
Hurd.
(bootstrap-executable-url): Use lilypond url for the Hurd.
(bootstrap-guile-url-path): Likewise.
(bootstrap-guile-hash): Add entry for the Hurd.
(%bootstrap-coreutils&co): Likewise.
(%bootstrap-binutils): Likewise.
(%bootstrap-glibc): Likewise.
(%bootstrap-gcc): Likewise.
* guix/packages.scm (%supported-systems): Add i586-gnu.

Co-authored-by: Jan Nieuwenhuizen <janneke@gnu.org>