Qualcomm: expose PAL headers via header map so QNN runtime builds under Buck

[Gasoonjia]

Summary

The PAL support added the pal/ sources and headers directly to the QNN runtime target and resolved #include <pal/...> with a hardcoded -Ibackends/qualcomm/runtime/pal/include preprocessor flag. That -I path is resolved relative to the Buck project root, so it does not resolve correctly across build roots and #include <pal/DynamicLoading.h> / <pal/Path.h> fail to compile when building the QNN runtime under Buck.

Fix

Move the PAL sources and headers into a dedicated pal library and expose the headers through a header map (a dict exported_headers with an empty header_namespace) instead of an -I flag. runtime picks it up via exported_deps, so dependents that include QnnImplementation.h resolve <pal/...> too. This mirrors what the CMake build already does with include_directories(runtime/pal/include), and does not disturb the runtime target's namespaced <executorch/...> exported headers.

No functional change — Buck build wiring only.

[pytorch-bot]

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❌ 2 New Failures, 2 Pending, 3 Unrelated Failures

As of commit 8929a43 with merge base 218cc45 ():

NEW FAILURES - The following jobs have failed:

• pull / test-phi-3-mini-runner-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 537926da13b764062451c6585dbed701b2611d6a8e91005d88f4ecbbf674db68 /exec failed with exit code 1
• pull / unittest / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_resnet50_model

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

BROKEN TRUNK - The following jobs failed but was present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / test-arm-backend-no-driver (test_pytest_models_tosa) / linux-job (gh) (trunk failure)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.
• pull / test-llama-runner-qnn-linux (fp32, qnn_16a16w, qnn) / linux-job (gh) (trunk failure)

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[psiddh]

Thanks @Gasoonjia for this

[psiddh]

cc @shewu-quic @haowhsu-quic @winskuo-quic @chenweng-quic