accel/tcg/plugin-gen: fix the call signature for inline callbacks

accel/tcg/plugin-gen: fix the call signature for inline callbacks

A recent change to the handling of constants in TCG changed the
pattern of ops emitted for a constant add. We no longer emit a mov and
the constant can be applied directly to the TCG_op_add arguments. This
was causing SEGVs when running the insn plugin with arg=inline. Fix
this by updating copy_add_i64 to do the right thing while also adding
a comment at the top of the append section as an aide memoir if
something like this happens again.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Cc: Emilio G. Cota <cota@braap.org>
Message-Id: <20210213130325.14781-10-alex.bennee@linaro.org>