disas: Add capstone as submodule

Do not require the submodule, but use it if present.  Allow the
command-line to override system or git submodule either way.

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>
