scripts: Add decodetree.py

To be used to decode ARM SVE, but could be used for any fixed-width ISA.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

tests/decode/err_init3.decode

@@ -0,0 +1,7 @@
+# This work is licensed under the terms of the GNU LGPL, version 2 or later.
+# See the COPYING.LIB file in the top-level directory.
+
+# Diagnose member initialized twice in pattern + format.
+&args	a
+@format	........ ........ a:16	&args
+insn	00000000 00000000 a:16	@format