LLVM 18 Lands -march=arrowlake / arrowlake-s / lunarlake

Going along with LLVM's recent additions around supporting new Intel instructions coming with future generation Core CPUs, the LLVM 18 Git development code has now landed support for actually honoring -march=arrowlake, -march=arrowlake-s, and -march=lunarlake targets.

Like with Intel working out the Arrow Lake and Lunar Lake support in GCC and the GNU compiler toolchain at large, LLVM has seen the new ISA capabilities added (AVX-VNNI-INT16, SHA512, SM3, and SM4) and now the appropriate -march= CPU microarchitecture targeting support for the Clang C/C++ compiler.

For now the Arrow Lake and Lunar Lake targets continue reusing the existing Alder Lake scheduler model and tuning. Arrow Lake S and Lunar Lake have the shiny new instructions while Arrow Lake (non-S) models have the same instruction set features as Sierra Forest.

This patch merged today to LLVM Git for the LLVM 18 release next spring adds in the -march= targeting for these 2024 and 2025 Intel Core CPUs.