tcg: Pass function pointer to tcg_gen_call*

tcg: Pass function pointer to tcg_gen_call*

For normal helpers, read the function pointer from the
structure earlier.  For plugins, this will allow the
function pointer to come from elsewhere.

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