[X86] Add 'znver2' and 'cascadelake' support to __cpu_indicator_init.

For 'cascadelake' this is adding a 'avx512vnni' feature check to the 0x55 skylake-avx512 model check. These CPUs use the same model number and only differ in the stepping number. But the feature flag is simpler than collecting all the stepping numbers.

For 'znver2' this is just syncing with LLVM's Host.cpp.

llvm-svn: 354927