[X86] Add HasCLFLUSH pseudo-predicate (Issue #19039)

Similar to what we've done for HasMFence - this puts into place a pseudo-predicate for CLFLUSH instructions that separates it from HasSSE2 to make it easier to use CLFLUSH even when SSE/fpmath has been disabled - technically CLFLUSH has its own CPUID bit, so could be available on x86 cores entirely without SSE, but I don't think thats ever happened or likely to happen.