Control for `smt` / `hyperthreading`

[jcpunk]

It would be nice if there was a simple abstraction layer I could use to enable/disable smt / hyperthreading as a part of a tuning profile.

Some of my realtime workloads have significant benefits from having smt disabled.

[adriaan42]

@jcpunk a while ago there was some discussion (and code) on how this could be done: #366