add up-front check and documentation for oldest supported x86 processor

Today there are some places in DR where we assume the processor has SSE, yet we didn't check for that at init time nor did we document this. We should add an up-front check with a nice message, and add to the Limitations docs, on what we support. I don't think it's worth adding runtime support for super-old processors by adding non-SSE branching in the cxt sw, etc. code. If someone is interested in doing that, that sounds fine, but we have limited developer time and it doesn't seem high priority.