Fix libffi config header preprocessor definition. (#9159)

  - The preprocessor variable FFI_EXEC_TRAMPOLINE_TABLE should be defined
    with a value. Either 0 or 1.

  - On almost all systems we supported so far this was undefined completely
    so it did not cause any issues.

  - With `macOS` on `aarch64` systems (like the M1 mac) libffi wants this to
    be "on". The CMake configuration was defining it to nothing, i.e.,

    ```c
    #define FFI_EXEC_TRAMPOLINE_TABLE
    ```

    and then compilation fails because the sources contain checks like

    ```c
    #if FFI_EXEC_TRAMPOLINE_TABLE
    ```
    which expect a value.

    Always define it either to 0 or 1. If it is to be "defined" it should
    be

    ```c
    #if FFI_EXEC_TRAMPOLINE_TABLE 1
    ```

    To be honest I am not sure if I completely understood how libffi wants
    to use this variable. But looking at the way their sources use this
    variable it seems like this is the right thing to do.

    I suspect the autoconf build also has this issue and should probably
    be fixed as well.

  - Improves #9145.

Author: mahge