scylla_prepare: verify processor satisfies instruction set requirements
scylla_prepare: verify processor satisfies instruction set requirements

Scylla requires the CLMUL and SSE 4.2 instruction sets and will fail without them.
There is a check in main(), but that happens after the code is running and it may
already be too late. Add a check in scylla_prepare which runs before the main
executable.

scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla