platform: add hack to use openstack fetcher for s390x qemu

s390x doesn't support the qemu fw_cfg mechanism. Let's hack around this
for now by having the fetcher on s390x just use the same one as
OpenStack, i.e. config drives (and technically metadata server too,
which will fail... again, this is a short-term hack).

For background, see:
coreos/coreos-assembler#1004
coreos/ignition-dracut#145
coreos#825

internal/platform/platform.go

@@ -16,6 +16,7 @@ package platform
 
 import (
 	"fmt"
+	"runtime"
 
 	"github.com/coreos/ignition/v2/internal/log"
 	"github.com/coreos/ignition/v2/internal/providers"
@@ -126,6 +127,10 @@ func init() {
 		name:  "qemu",
 		fetch: qemu.FetchConfig,
 	})
+	configs.Register(Config{
+		name:  "qemu_s390x",
+		fetch: openstack.FetchConfig,
+	})
 	configs.Register(Config{
 		name:  "file",
 		fetch: file.FetchConfig,
@@ -150,7 +155,10 @@ func Get(name string) (config Config, ok bool) {
 }
 
 func MustGet(name string) Config {
-	if config, ok := Get(name); ok {
+	// try to get arch-specific fetcher first
+	if config, ok := Get(fmt.Sprintf("%s_%s", name, runtime.GOARCH)); ok {
+		return config
+	} else if config, ok := Get(fmt.Sprintf(name)); ok {
 		return config
 	} else {
 		panic(fmt.Sprintf("invalid platform name %q provided", name))