Add EXECUTORCH_SEPARATE_FLATCC_HOST_PROJECT cmake option (#3356)

[dbort]

Summary:
Currently, we always build two copies of the flatcc targets, just in case we happen to be cross-compiling. But because the flatcc project puts its binaries in the source directory, those two copies can interfere with each other.

We don't need to build two copies when not cross-compiling, so add a new option to avoid the second "host" build.

Eventually we should only enable this when cross-compiling, but for now disable it when building the pip package (which is never cross-compiled).

Pull Request resolved: #3356

Test Plan: rm -rf pip-out && ./install_requirements.sh succeeded. Looking in the pip-out/temp.*/cmake-out directory, there is no _host_build directory, but the etdump headers were successfully generated under pip-out/temp.*/cmake-out/sdk/include/executorch/sdk/etdump/.

Reviewed By: malfet, larryliu0820

Differential Revision: D56582507

Pulled By: dbort

fbshipit-source-id: 4ce6c680657bc57cfcf016826364a3f46c4c953e

[pytorch-bot]

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❌ 6 New Failures

As of commit 926d70e with merge base d3326a2 ():

NEW FAILURES - The following jobs have failed:

• trunk / test-coreml-delegate / macos-job (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1
• trunk / test-models-macos (cmake, emformer_join, portable, macos-m1-stable, 90) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• trunk / test-models-macos (cmake, emformer_predict, portable, macos-m1-stable, 90) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• trunk / test-models-macos (cmake, emformer_transcribe, portable, macos-m1-stable, 90) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• trunk / test-models-macos (cmake, w2l, portable, macos-m1-stable, 90) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• trunk / test-models-macos (cmake, w2l, xnnpack-delegation, macos-m1-stable, 90) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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