Align MSVC hardening with ONNX Runtime Spectre runtime-library handling

[Copilot]

ONNX_HARDENING=ON already enabled /Qspectre on MSVC, but it did not ensure the link step also picked the Spectre-mitigated CRT/STL libraries. This mirrors the relevant part of the upstream ONNX Runtime change so Windows hardening builds do not advertise Spectre mitigation while still linking the default runtime libraries.

• MSVC hardening

  • adds CMake helpers to resolve the Spectre library directory from VCToolsInstallDir
  • normalizes target architecture names (x86, x64, arm, arm64, arm64ec)
  • when /Qspectre is active, prepends lib/spectre/<arch> to the linker search path
  • falls back from arm64ec to arm64 when that is how the toolset ships the Spectre libs

• Failure mode

  • emits a targeted configure-time warning when /Qspectre is enabled but the Spectre-mitigated MSVC runtime libraries are not installed

• Documentation

  • updates the C++ linking / hardening docs to describe the Windows-specific /Qspectre behavior

Example of the new behavior in the hardening path:

if(_onnx_light_msvc_spectre_lib_dir)
  list(APPEND _onnx_light_hardening_link_options
       "SHELL:/LIBPATH:\"${_onnx_light_msvc_spectre_lib_dir}\"")
endif()