runtime: add padding to FFI type of struct ending with zero-sized field

runtime: add padding to FFI type of struct ending with zero-sized field

    CL 157557 changes the compiler to add one byte padding to
    non-empty struct ending with a zero-sized field. Add the same
    padding to the FFI type, so reflect.Call works.

    This fixes test/fixedbugs/issue26335.go in the main repo.

    Reviewed-on: https://go-review.googlesource.com/c/158018

From-SVN: r267956