CMake misclassifies IntelLLVM as GCC, causing unsupported warning and mitigation flags

[NLeeson]

IntelLLVM is routed through GCC-only warning and Spectre-mitigation options.
With icx/icpx 2026.1.1, this fails under -Werror:

error: unknown warning option '-Wno-stringop-overflow'; did you mean '-Wno-shift-overflow'? [-Werror,-Wunknown-warning-option]

The GCC mitigation options also produce unsupported-option diagnostics.

Reproduction

Linux x86-64, CMake 4.4.3, compute-runtime commit
05b847e3d2e72a6d68813a876a29b852db28200d, with build dependencies installed:

cmake -S . -B build-icpx \
  -DCMAKE_C_COMPILER=icx \
  -DCMAKE_CXX_COMPILER=icpx \
  -DCMAKE_BUILD_TYPE=Release \
  -DNEO_SKIP_UNIT_TESTS=ON \
  -DNEO_DISABLE_LTO=ON
cmake --build build-icpx --target cpp_generate_tool -j2

Expected: compiler options compatible with IntelLLVM.

The fix in #1008 was validated with a successful full driver build.