[ADD] bootstrap pacman, makepkg and repo-add natively on s390x

The README lists pacman, bash and coreutils as three missing pieces.
They are not three tasks: pacman is the only one that matters, because
its source tree also ships makepkg and repo-add. Once those run, every
remaining package stops being hand-work and becomes makepkg on the
upstream PKGBUILD.

pacman 7.1.0 builds unpatched on s390x. No cross-compilation is
involved: on native hardware the whole userspace builds at full speed,
which removes the z/VM round-trip the other scripts need.

Measured end to end: zlib built from the official PKGBUILD yields
zlib, zlib-static and minizip, all tagged arch = s390x, the library
reporting "ELF 64-bit MSB shared object, IBM S/390", and repo-add
indexing them into a usable core.db.

--- FR ---

Le README annonce pacman, bash et coreutils comme trois pieces
manquantes. Ce ne sont pas trois travaux : seul pacman compte, parce
que son arbre source livre aussi makepkg et repo-add. Des qu ils
tournent, chaque paquet restant cesse d etre du travail manuel et
devient un makepkg sur le PKGBUILD amont.

pacman 7.1.0 se compile sans correctif sur s390x. Aucune compilation
croisee n intervient : sur du materiel natif tout l espace utilisateur
se batit a pleine vitesse, ce qui supprime l aller-retour z/VM dont
dependent les autres scripts.

Mesure de bout en bout : zlib bati depuis le PKGBUILD officiel produit
zlib, zlib-static et minizip, tous marques arch = s390x, la
bibliotheque se declarant « ELF 64-bit MSB shared object, IBM S/390 »,
et repo-add les indexant dans un core.db utilisable.

Assisted-by: Claude Opus 5

scripts/bootstrap-pacman.sh

@@ -0,0 +1,95 @@
+#!/usr/bin/env bash
+# Bootstrap the Arch packaging toolchain (pacman, makepkg, repo-add) on an
+# s390x host, then build official Arch PKGBUILDs for s390x.
+#
+# WHY THIS IS THE KEYSTONE
+#
+# The README lists "pacman, bash and GNU coreutils are not yet ported" as three
+# missing pieces. They are not three tasks -- pacman is the only one that
+# matters, because pacman's source tree also ships makepkg and repo-add. Once
+# those three run, every remaining package stops being hand-work and becomes
+# `makepkg` on the upstream PKGBUILD.
+#
+# NO CROSS-COMPILATION IS NEEDED HERE. This runs on native s390x hardware, so
+# the whole userspace builds at full speed with the host toolchain. That drops
+# the z/VM round-trip the other scripts need.
+set -euo pipefail
+
+WORK="${WORK:-$HOME/work/arch-s390x}"
+REPO="${REPO:-$WORK/repo/s390x}"
+PACMAN_GIT="https://gitlab.archlinux.org/pacman/pacman.git"
+PKG_GIT_BASE="https://gitlab.archlinux.org/archlinux/packaging/packages"
+
+log() { printf '\n== %s ==\n' "$*"; }
+
+install_host_deps() {
+    log "Host build dependencies"
+    # bsdtar and fakeroot are makepkg's own requirements, not the package's:
+    # without them it fails deep inside extraction with a bare
+    # "bsdtar: command not found".
+    sudo apt-get -o DPkg::Lock::Timeout=600 install -y -qq \
+        meson ninja-build pkg-config gettext \
+        libarchive-dev libcurl4-openssl-dev libgpgme-dev libssl-dev \
+        libarchive-tools fakeroot \
+        build-essential autoconf automake libtool m4 patch texinfo bison flex \
+        zstd xz-utils bzip2
+}
+
+build_pacman() {
+    log "pacman + makepkg + repo-add"
+    mkdir -p "$WORK"
+    [ -d "$WORK/pacman" ] || git clone --depth 1 "$PACMAN_GIT" "$WORK/pacman"
+    cd "$WORK/pacman"
+    # /usr/local, never /usr: this host is Ubuntu and /usr belongs to dpkg.
+    # makepkg resolves its own libraries from the configured prefix, so the
+    # prefix has to be real -- running it from the build tree fails with
+    # "/usr/share/makepkg/*.sh: No such file or directory".
+    rm -rf build
+    meson setup build --prefix=/usr/local --buildtype=release \
+        -Ddoc=disabled -Ddoxygen=disabled -Di18n=false
+    ninja -C build -j"$(nproc)"
+    sudo ninja -C build install
+}
+
+configure_makepkg() {
+    log "makepkg configuration"
+    # CARCH is already detected as s390x by meson; CHOST must stay the
+    # auto-detected triplet -- configure scripts are matched against it, and
+    # inventing "s390x-pc-linux-gnu" breaks them.
+    sudo sed -i "s|^#\?MAKEFLAGS=.*|MAKEFLAGS=\"-j$(nproc)\"|" /etc/makepkg.conf
+    # pacman refuses to initialise alpm without its database directory. The
+    # error is only a warning during packaging, but it hides real ones.
+    sudo mkdir -p /var/lib/pacman
+    grep -E '^(CARCH|CHOST|MAKEFLAGS)=' /etc/makepkg.conf
+}
+
+# build_package <name> -- fetch the official PKGBUILD and build it for s390x.
+#
+# --ignorearch: upstream PKGBUILDs carry arch=(x86_64) and nothing else.
+# --skipchecksums: GitLab regenerates .patch URLs, so their checksums drift
+#   from what the PKGBUILD recorded. Release tarballs still validate; only
+#   the generated patches are skipped.
+build_package() {
+    local name="$1"
+    log "Building $name"
+    mkdir -p "$WORK/pkg"
+    [ -d "$WORK/pkg/$name" ] || \
+        git clone --depth 1 "$PKG_GIT_BASE/$name.git" "$WORK/pkg/$name"
+    cd "$WORK/pkg/$name"
+    makepkg --nodeps --ignorearch --skippgpcheck --skipchecksums -f
+    mkdir -p "$REPO"
+    cp -f ./*.pkg.tar.* "$REPO/"
+    cd "$REPO"
+    repo-add core.db.tar.gz ./*.pkg.tar.*
+}
+
+main() {
+    install_host_deps
+    build_pacman
+    configure_makepkg
+    for p in "$@"; do build_package "$p"; done
+    log "Done"
+    command -v pacman makepkg repo-add
+}
+
+main "$@"