kernel - Flesh out Spectre mitigation support

kernel - Flesh out Spectre mitigation support

* Add handling for all modes for AMD CPUs, including support for
  IBRS_AUTO and STIBP_AUTO (always-on) bits which will be added
  to future cpus.

* Add STIBP handling to Intel CPUs.  I can't find definitions for
  AUTO (always-on) modes for Intel, so those are still not supported
  (no current CPU has AUTO support yet anyway).

* Current DragonFlyBSD defaults: Will enable IBRS_AUTO and STIBP_AUTO
  (always on) support by default if the cpu has it.  Will NOT enable IBRS
  or STIBP (non-auto) toggling by default.  Will not enable IBPB by default.

  IBPB is currently not enabled by default.  The overhead is an enormous
  ~2uS.  We will follow Linux in this regard.

* Change the machdep.spectre_mitigation sysctl to take a string of
  features to enable.  Change machdep.spectre_support to display a
  string of features supported.  Possible features are:

  IBRS          Indirect Branch Restricted Speculation (U->K and K->U)
  STIBP         Single Thread Indirect Branch Prediction (U->K and K->U)
  IBPB          Branch Prediction Barrier (U->K)
  IBRS_AUTO     IBRS always-on (set once and forget)
  STIBP_AUTO    STIBP always-on (set once and forget)

  The machdep.spectre_support sysctl tells you whats available.

* Refactor tr_pcb_gflags into tr_pcb_spec_ctrl[2] to make it easier
  for the assembly code to program the SPEC_CTRL MSR.

* Note that some of the above bits will never be supported by current
  hardware and exist to allow future hardware to support these features
  in a less expensive manner.

* Also note that for Meltdown, AMD is immune and the meltdown mitigation
  will not be enabled.  Intel is vulnerable and the mitigation will be
  enabled by default.  See sysctl machdep.meltdown_mitigation.