Make flatcc cross-compile deterministic

[dbort]

(clone of @haowhsu-quic's #4312)

As mentioned in https://github.com/pytorch/executorch/pull/4297, the original flow makes host / cross build happen concurrently. This change moves host build process into cmake configuring stage and refine related dependencies.

Test Plan:

• cross-compile > Through running backends/qualcomm/script/build.sh --release, we could check if the compiling process successfully finished.
• native-compile > Run following to check:

cmake \
    -DCMAKE_BUILD_TYPE=RelWithDebInfo \
    -DQNN_SDK_ROOT=${QNN_SDK_ROOT} \
    -DEXECUTORCH_BUILD_QNN=ON \
    -DEXECUTORCH_BUILD_SDK=ON \
    -DPYTHON_EXECUTABLE=$PYTHON_EXECUTABLE \
    -S $EXECUTORCH_ROOT \
    -B $EXECUTORCH_ROOT/build_x86_64 \

[pytorch-bot[bot]]

:link: Helpful Links

:test_tube: See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/4515

• :page_facing_up: Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

:white_check_mark: No Failures

As of commit 77136667e7b4b06af1b264f54fd9346c3e949e02 with merge base d59419c4d56f7f14b3e6ac65848ce23c1b3ee108 (): :green_heart: Looks good so far! There are no failures yet. :green_heart:

This comment was automatically generated by Dr. CI and updates every 15 minutes.