Enable buck-native x86 simulator test for QNN op tests

[billmguo]

Summary:
The QNN operator tests in backends/qualcomm/tests/test_qnn_delegate.py already support host x86_64 simulator execution via --enable_x86_64, but were only runnable as a standalone argparse script after a CMake build. There was no buck test path: test_qnn_delegate is a python_library (not a test target), and the host qnn_executor_runner binary was Android-only.

This change wires up a buck-native, internal-only test target //executorch/backends/qualcomm/tests:test_qnn_delegate_x86 that runs the FP16 and quantized operator suites (TestQNNFloatingPointOperator, TestQNNQuantizedOperator) on the x86 QNN simulator with no device and no CMake build tree.

Changes:

• backends/qualcomm/runtime/targets.bzl: add CXX to the pal library platforms so they match :logging and :runtime (both [ANDROID, CXX]). :pal ships pal/src/linux/*.cpp and is an exported dep of :runtime, so its host (CXX) variant must exist for the :runtime CXX build to resolve on Linux -- both the OSS buck2 build //backends/qualcomm/... job (test-qnn-buck-build-linux) and the internal x86 simulator runner. (Not gated on is_fbcode(): the OSS Linux job builds the :runtime CXX variant too, so pal must provide CXX there as well.)
• examples/qualcomm/executor_runner/targets.bzl: add CXX to qnn_executor_runner (gated if is_fbcode()) so the host runner binary builds, and add //executorch/kernels/portable:generated_lib to its deps. The CMake runner links full_portable_ops_lib + quantized_ops_lib; the Buck runner had only the quantized lib, so ops that leave a CPU-fallback node (e.g. acos -> aten::asin.out, cast, index_copy, index_put, logical_and, avg_pool1d) aborted the runner with a "Missing operator" error. The CXX (host) surface is gated to is_fbcode() because it is only used by the internal x86 simulator test; in OSS, CXX includes macOS (no QNN host libs), so the host runner/pal stay Android-only there, restoring the original OSS build surface.
• backends/qualcomm/tests/test_qnn_delegate_x86.py (new): under buck test the file's argparse __main__/setup_environment() never runs, so this wrapper sets the equivalent TestQNN class attributes (enable_x86_64, backend, soc_model) at import and subclasses the operator TestCases so the runner discovers them.
• backends/qualcomm/tests/BUCK: add the test_qnn_delegate_x86 python_test, gated behind runtime.is_oss via a top-level conditional expression (the BUCK dialect forbids top-level if/def). It stays out of the OSS graph to preserve the test-qnn-buck-build-linux CI signal. The QNN x86 SDK libs and host runner are supplied via env (QNN_SDK_ROOT, LD_LIBRARY_PATH, QNN_EXECUTOR_RUNNER). CUDA_VISIBLE_DEVICES is forced empty because these tests are CPU-only (calibration, AOT compile, x86 sim); without it, parallel test shards each grabbed CUDA and exhausted GPU memory.

The fbcode and xplat copies of all four files are kept byte-identical per the existing twin convention.

Reviewed By: YIWENX14

Differential Revision: D109606746

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