Emulate AVX-512 on Linux runners that lack it

[bfierz]

Summary

• The Linux AVX-512 legs (ubuntu-latest, gcc-12/clang-18) previously just skipped ctest when the assigned runner's CPU didn't support AVX-512, so those legs frequently didn't actually exercise the AVX-512 backend at all.
• They now fall back to running under Intel SDE (sde64 -skx) when /proc/cpuinfo shows the host lacks avx512f/avx512vl/avx512dq, via CMAKE_CROSSCOMPILING_EMULATOR — the same mechanism this workflow already uses for the armv7 qemu-arm cross job, just pointed at a different emulator. SDE (downloaded, checksum-pinned) is a dynamic binary instrumentation tool built by Intel specifically to emulate exact microarchitectures, including newer ISA extensions on hardware that predates them; -skx targets Skylake-X, the first mainstream chip with AVX-512F/CD/BW/DQ/VL.
  • QEMU's qemu-x86_64 user-mode TCG emulator was tried first (same CMAKE_CROSSCOMPILING_EMULATOR mechanism, -cpu max), but its AVX-512 coverage proved too incomplete for this codebase's instruction mix: most tests segfaulted under it, inconsistently (some passed, most didn't), pointing at gaps in specific instruction forms rather than a config mistake.
• When the runner does have native AVX-512 support, nothing changes: the emulator arg stays empty and tests run natively as before (no emulation overhead).
• Windows AVX-512 is untouched — it still skips on unsupported hardware, since this only covers the Linux legs.