internal/rosa/stage0: add riscv64 tarball

This had not yet passed all test suites because emulator is prohibitively slow.

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

internal/rosa/rosa.go

@@ -56,6 +56,8 @@ func linuxArch() string {
 		return "x86_64"
 	case "arm64":
 		return "aarch64"
+	case "riscv64":
+		return "riscv64"
 
 	default:
 		panic("unsupported target " + runtime.GOARCH)