Add upstream flatcc option to avoid installing under the source tree

The flatcc third party library builds its libraries and executables directly into its source tree (under bin/ and lib/) instead of into the cmake output directory. This causes problems for us, because we need to be able to build both host and cross-compiled target versions of flatcc. But since they both are built to the same directory, they can overwrite each other.

We have a hack at https://github.com/pytorch/executorch/blob/4f12131c0c57d903093b4d26f9eccea1f9545222/devtools/CMakeLists.txt#L105 to deal with this, but it'd be a lot easier if flatcc had a build option to install the binaries under the cmake build directory instead.

@dbort sent https://github.com/dvidelabs/flatcc/pull/306 to the upstream project, but there's been no further response for months.

https://github.com/pytorch/executorch/commit/db31f4961e8a576af93af34dbbc43965ee1edec9 is the simplification change we'll be able to make to ET. And that hack didn't work all the time anyway (e.g., #7260), so with this we should be able to permanently avoid the environment-specific build race conditions we've run into.

cc @larryliu0820 @lucylq