TFLite: get closer to build with Bazel on Windows (#14791)

Bazel cannot yet build TensorFlow Lite on Windows,
but this commit gets us closer.

In this commit:
- make the -Wno-implicit-fallthrough compiler flag
  in flatbuffers' BUILD file be conditional to
  non-Windows builds, because MSVC doesn't know
  this flag
- fix the Bazel build command in README.md by
  removing single quotes around --cxxflags,
  because it's not needed on Bash and is harmful
  on Windows (because cmd.exe doesn't remove the
  single quotes)
- fix non-ASCII quotes and apostrophes, as well as
  some formatting issues in README.md

See https://github.com/bazelbuild/bazel/issues/4148