cmd/mbf: optional emulated target architecture

This enables transparent cross-compilation without breaking purity.

Signed-off-by: Ophestra <cat@gensokyo.uk>

cmd/mbf/main.go

@@ -100,6 +100,10 @@ func main() {
 		&flagQuiet,
 		"q", command.BoolFlag(false),
 		"Do not print cure messages",
+	).Flag(
+		&cm.arch,
+		"arch", command.StringFlag(runtime.GOARCH),
+		"Target architecture",
 	).Flag(
 		&flagLTO,
 		"lto", command.BoolFlag(false),