gnu: bootstrap: Add support for riscv64-linux.

gnu: bootstrap: Add support for riscv64-linux.

On 7d93b21ab1c132990054372a9677c1639d54e631
    gnu: glibc-for-bootstrap: Update patch.

Run
    ./pre-inst-env guix build --target=riscv64-linux-gnu bootstrap-tarballs

Producing
    /gnu/store/4hdzva9i0wyyfbgj1lmqc1wkk644pv07-bootstrap-tarballs-0

With guix hash -rx
    1nj0fdgj08bbmfny01mp2blv7c3p2iciqh31zmf04ap5s7ygsqlp

* gnu/packages/bootstrap.scm (%bootstrap-executables): Add entries for
riscv64-linux.
(%bootstrap-guile-hash, %bootstrap-coreutils&co, %bootstrap-binutils,
%bootstrap-glibc, %bootstrap-gcc): Add entry for riscv64-linux.
(raw-build-guile3): New procedure.
(make-raw-bag): Use raw-build-guile3 for riscv64-linux.
* guix/packages.scm (%supported-systems): Add riscv64-linux.
(%cuirass-supported-systems): Remove riscv64-linux.
* guix/utils.scm (target-64bit?): Add riscv64-linux.
* m4/guix.m4: Add riscv64-linux as a supported system.
* doc/guix.texi (GNU Distribution): Add riscv64-linux.