make release: add cross-build

This implements cross-build for "make release", moving the build into a
container. This way we can support arm, arm64, ppc, and whatnot.

* script/seccomp.sh: separate out of script/release.sh, amend to support
  cross-compile and save needed environment variables to a file.

* Dockerfile: add installing libseccomp from source, as this is needed
  for release builds.

* script/release.sh: amend to support more architectures in addition to
  the native build. Additional arches can be added by specifying
  "-a <arch>" argument (can be specified multiple times), or
  "make RELEASE_ARGS="-a arm64" release" if called via make.
  All supported architectures can be enabled via "make releaseall".

* Makefile: move "release" target to "localrelease", add "release" and
  "releaseall" targets to build via the Dockerfile. This is done because
  most distros (including Fedora and openSUSE) lack cross-glibc, which is
  needed to cross-compile libseccomp.

* Makefile: remove 'cross' and 'localcross' targets, as this is now done
  by the release script.

* .github/workflows/validate.yum: amend the release CI job to cross-build
  for supported architectures, remove cross job.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

.github/workflows/validate.yml

@@ -124,27 +124,6 @@ jobs:
           error: 'Subject too long (max 72)'
 
 
-  cross:
-    runs-on: ubuntu-20.04
-    steps:
-    - uses: actions/checkout@v2
-      # We have to run this under Docker as Ubuntu (host) does not support all
-      # the architectures we want to compile test against, and Dockerfile uses
-      # Debian (which does).
-      #
-      # XXX: as currently this is the only job that is using Docker, we are
-      # building and using the runcimage locally. In case more jobs running
-      # under Docker will emerge, it will be good to have a separate make
-      # runcimage job and share its result (the docker image) with whoever
-      # needs it.
-    - uses: satackey/action-docker-layer-caching@v0.0.11
-      continue-on-error: true
-    - name: build docker image
-      run: make runcimage
-    - name: cross
-      run: make cross
-
-
   cfmt:
     runs-on: ubuntu-20.04
     steps:
@@ -169,12 +148,21 @@ jobs:
       uses: actions/checkout@v2
       with:
         fetch-depth: 0
-    - name: install deps
-      run: |
-        sudo apt -qq update
-        sudo apt -qq install gperf
-    - name: make release
-      run: make release
+      # We have to run this under Docker as Ubuntu (host) does not support all
+      # the architectures we want to compile test against, and Dockerfile uses
+      # Debian (which does).
+      #
+      # XXX: as currently this is the only job that is using Docker, we are
+      # building and using the runcimage locally. In case more jobs running
+      # under Docker will emerge, it will be good to have a separate make
+      # runcimage job and share its result (the docker image) with whoever
+      # needs it.
+    - uses: satackey/action-docker-layer-caching@v0.0.11
+      continue-on-error: true
+    - name: build docker image
+      run: make runcimage
+    - name: make releaseall
+      run: make releaseall
     - name: upload artifacts
       uses: actions/upload-artifact@v2
       with: