[WIP] pkg/{flatbuffers,tflite-micro}: upgrade & fix compilation with LLVM

[maribu]

Contribution description

• upgrade pkg/flatbuffers and re-enable building with LLVM
• always use flatc from dist to avoid version mismatch between headers and flatc
• fix pkg/tflite-micro to build with new pkg/flatbuffers

WARNING:

• This still includes a commit to disable LLVM, which I expect to land in master soonish. This needs to be rebased on top of master once this is in.
• Fixing pkg/tflite-micro to build with pkg/flatbuffers is not yet done

Testing procedure

Both packages should build with both GCC and LLVM and the corresponding test apps should pass.

Issues/PRs references

Split out of #19634