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>

script/tmpmount

@@ -1,4 +0,0 @@
-#!/bin/bash
-
-mount -t tmpfs none /tmp
-exec "$@"