This adds support for 64-bit instructions to decodetree.py.

It will be necessary to later on use decodetree to implement
the new 64-bit Power ISA 3.1 instructions.

While doing this change, I thought it would also be nice to be
able to specify different sizes for each field in arg structs and
also infer whether to use signed/unsigned data types based on
the field definition. But those would be different changes,
anyway, and I limited myself to using int64_t for the data type
of fields in arg structs when insnwidth == 64.

Luis Pires (1):
  decodetree: Add support for 64-bit instructions

 docs/devel/decodetree.rst |  3 ---
 scripts/decodetree.py     | 25 ++++++++++++++++++++-----
 2 files changed, 20 insertions(+), 8 deletions(-)

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2.25.1