[llvm-exegesis][X86] Explicitly not support `WRFSBASE` opcode.

We can't serialize if for latency measurements, and when measuring uops,
it crashes so hard even `CrashRecoveryContext` doesn't stop it.

Looks like *this* was the last crasher, now `--opcode-index=-1`
succeeds for all three benchmark modes here.
At least with Duplication repetitor.