tcg: Build ffi data structures for helpers

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>

tests/docker/dockerfiles/debian10.docker

@@ -26,6 +26,7 @@ RUN apt update && \
         gdb-multiarch \
         gettext \
         git \
+        libffi-dev \
         libncurses5-dev \
         ninja-build \
         pkg-config \