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>

script/lib.sh

@@ -0,0 +1,36 @@
+#!/bin/bash
+
+# set_cross_vars sets a few environment variables used for cross-compiling,
+# based on the architecture specified in $1.
+function set_cross_vars() {
+	GOARCH="$1" # default, may be overridden below
+	unset GOARM
+
+	case $1 in
+	arm64)
+		HOST=aarch64-linux-gnu
+		;;
+	armel)
+		HOST=arm-linux-gnueabi
+		GOARCH=arm
+		GOARM=6
+		;;
+	armhf)
+		HOST=arm-linux-gnueabihf
+		GOARCH=arm
+		GOARM=7
+		;;
+	ppc64le)
+		HOST=powerpc64le-linux-gnu
+		;;
+	*)
+		echo "set_cross_vars: unsupported architecture: $1" >&2
+		exit 1
+		;;
+	esac
+
+	CC=$HOST-gcc
+	STRIP=$HOST-strip
+
+	export HOST GOARM GOARCH CC STRIP
+}