decodetree: Initial support for variable-length ISAs

Assuming that the ISA clearly describes how to determine
the length of the instruction, and the ISA has a reasonable
maximum instruction length, the input to the decoder can be
right-justified in an appropriate insn word.

This is not 100% convenient, as out-of-line %fields are
numbered relative to the maximum instruction length, but
this appears to still be usable.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>