feat: add riscv64 as supported arch

this makes kernel+squashfs flow work for risc64

Signed-off-by: Carsten Munk <carsten@zippie.com>

src/cmd/linuxkit/util/arch.go

@@ -9,6 +9,8 @@ func MArch(in string) (string, error) {
 		return "x86_64", nil
 	case "aarch64", "arm64":
 		return "aarch64", nil
+	case "riscv64":
+		return "riscv64", nil
 	}
 	return "", fmt.Errorf("unknown arch %q", in)
 }