tools: toolchain: prepare: build arch images in parallel

To speed up the build, run each arch in parallel, using bash's
awkward job control.

tools/toolchain/prepare

@@ -45,13 +45,31 @@ done
 buildah manifest create "$(<tools/toolchain/image)"
 
 
-for arch in "${archs[@]}"; do
+build_arch() {
+    local arch="$1"
     image_id_file="$(mktemp)"
     buildah bud --arch="$arch" --no-cache --pull -f tools/toolchain/Dockerfile --iidfile "$image_id_file"
     buildah manifest add --all "$(<tools/toolchain/image)" "$(<$image_id_file)"
     rm "$image_id_file"
+}
+
+for arch in "${archs[@]}"; do
+    build_arch "$arch" &
 done
 
+fails=0
+
+for arch in "${archs[@]}"; do
+    if ! wait -n; then
+        (( fails += 1 ))
+    fi
+done
+
+if (( fails > 0 )); then
+    echo "$fails failures building arch-specific images"
+    exit 1
+fi
+
 echo "Done building $(<tools/toolchain/image). You can now test it, and push with"
 echo ""
 echo "    podman manifest push --all $(<tools/toolchain/image) docker://$(<tools/toolchain/image)"