ci: move cross compile check from travis to gha

In here we have to use Docker, as Ubuntu does not support
all the architectures we're compile-testing here.

Since this is the only step that is using Docker,
there is no sense to separate `make runcimage` from
the rest of it. In case we'll have to use Docker image
more, it will make sense to do so.

While at it, ditch script/tmpmount (added by commit 1735ad788f),
because
 - it required root (because mount);
 - it is probably no longer needed.

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

.github/workflows/validate.yml

@@ -98,3 +98,22 @@ jobs:
           commits: ${{ steps.get-pr-commits.outputs.commits }}
           pattern: '^.{0,72}(\n.*)*$'
           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.
+    - name: build docker image
+      run: make runcimage
+    - name: cross
+      run: make cross