target/arm: explicitly encode regnum in our XML

This is described as optional but I'm not convinced of the numbering
when multiple target fragments are sent.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

Message-Id: <20200316172155.971-17-alex.bennee@linaro.org>