Add libffi as a build requirement for TCI.
Add libffi to the dockerfiles to satisfy that requirement.

Construct an ffi_cif structure for each unique typemask.
Record the result in a separate hash table for later lookup;
this allows helper_table to stay const.

Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>