Support for Pre-SSE2 CPUs

[Postrediori]

Almost every media about Spectre mentions CPUs starting from 1995 are vulnerable. This PoC uses rdtscp and clflush instructions that are not available on everything that was released before SSE2.

While it is possible to make workaround for rdtscp (e.g. like this comment on the original gist), it would be also interesting to create PoC for older chips with other means of flushing the cache, to find out the real age of the issue.

[crozone]

rdtsc is now used when the project is built with CFLAGS=-DNORDTSCP make (Pull request #2). This increases compatibility with Core 2 Duo and non x86-64 cores.

Still need to find a workaround for clflush, and place it behind a DNOCLFLUSH ifdef.

[crozone]

Added issue for clflush workaround: #6

[crozone]

Should be closed by pull request #7