Linux containers on FreeBSD

This allows running Linux containers on FreeBSD and modifies the
mounts so that they represent the linux emulated filesystems, as per:
https://wiki.freebsd.org/LinuxJails

Co-authored-by: Gijs Peskens <gijs@peskens.net>, Samuel Karp <samuelkarp@users.noreply.github.com>
Signed-off-by: Artem Khramov <akhramov@pm.me>

platforms/defaults_freebsd.go

@@ -0,0 +1,42 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package platforms
+
+import (
+	specs "github.com/opencontainers/image-spec/specs-go/v1"
+	"runtime"
+)
+
+// DefaultSpec returns the current platform's default platform specification.
+func DefaultSpec() specs.Platform {
+	return specs.Platform{
+		OS:           runtime.GOOS,
+		Architecture: runtime.GOARCH,
+		// The Variant field will be empty if arch != ARM.
+		Variant: cpuVariant(),
+	}
+}
+
+// Default returns the default matcher for the platform.
+func Default() MatchComparer {
+	return Ordered(DefaultSpec(), specs.Platform{
+		OS:           "linux",
+		Architecture: runtime.GOARCH,
+		// The Variant field will be empty if arch != ARM.
+		Variant: cpuVariant(),
+	})
+}

platforms/defaults_unix.go

@@ -1,5 +1,5 @@
-//go:build !windows && !darwin
-// +build !windows,!darwin
+//go:build !windows && !darwin && !freebsd
+// +build !windows,!darwin,!freebsd
 
 /*
    Copyright The containerd Authors.