feat(bootstrap_sdk): Build cross toolchains

Until now we haven't built our own cross toolchains and used old
packages from Google. It is time to break free and move on!

lib/catalyst_sdk_stage4.sh

@@ -2,5 +2,35 @@
 
 source /tmp/chroot-functions.sh
 
+# Build cross toolchains
+# crossdev only does full bootstraps so if all of the packages are already
+# installed (i.e. we are updating an existing stage4) then use emerge
+for cross_chost in x86_64-cros-linux-gnu; do
+    echo "Installing toolchain for ${cross_chost}"
+    cross_pkgs=( cross-${cross_chost}/{binutils,gcc,gdb,glibc,linux-headers} )
+    cross_bootstrap=0
+    for pkg in "${cross_pkgs[@]}"; do
+        if ! portageq match / "$pkg" | grep .; then
+            cross_bootstrap=1
+            break
+        fi
+    done
+
+    if [[ "${cross_bootstrap}" -eq 1 ]]; then
+        crossdev --ov-output "/usr/local/portage/crossdev" \
+            --portage "${clst_myemergeopts}" \
+            --env 'FEATURES=splitdebug' \
+            --stable --ex-gdb --stage4 \
+            --target "${cross_chost}" || exit 1
+    else
+        # Still run --init-target to ensure config is correct
+        crossdev --ov-output "/usr/local/portage/crossdev" \
+            --env 'FEATURES=splitdebug' \
+            --stable --ex-gdb --init-target \
+            --target "${cross_chost}" || exit 1
+        run_merge -u "${cross_pkgs[@]}"
+    fi
+done
+
 echo "Double checking everything is fresh and happy."
 run_merge -uDN --with-bdeps=y world