Add generic "t64" and "t64-systemd" aliases

Add generic "t64" and "t64-systemd" aliases that cover all images with
64-bit time_t: both t64 images for 32-bit architectures and the regular
images for 64-bit architectures.  This makes it possible to use:

    FROM gentoo/stage3:t64

to get a system with 64-bit time_t without having to separately cover
individual architectures.
Closes: #150

Signed-off-by: Michał Górny <mgorny@gentoo.org>

.github/workflows/build.yml

@@ -97,6 +97,8 @@ jobs:
           - stage3:ssemath-t64
           - stage3:ssemath-t64-systemd
           - stage3:systemd
+          - stage3:t64
+          - stage3:t64-systemd
     name: ${{ matrix.target }}
     runs-on: ubuntu-latest
     steps:

deploy-manifests.sh

@@ -54,6 +54,12 @@ case "${TARGET}" in
 	"stage3:systemd")
 	    TAGS=("amd64-systemd" "armv5tel-systemd" "armv6j_hardfp-systemd" "armv7a_hardfp-systemd" "arm64-systemd" "i686-systemd" "ppc64le-systemd" "rv64_lp64d-systemd")
 		;;
+	"stage3:t64")
+	    TAGS=("amd64-openrc" "arm64-openrc" "i686-ssemath-t64-openrc" "ppc64le-openrc" "rv64_lp64d-openrc" "s390x")
+		;;
+	"stage3:t64-systemd")
+	    TAGS=("amd64-systemd" "arm64-systemd" "i686-ssemath-t64-systemd" "ppc64le-systemd" "rv64_lp64d-systemd")
+		;;
 	*)
 		echo "Done! No manifests to push for TARGET=${TARGET}."
 	    exit 0