kernel - Implement spectre mitigations part 1

kernel - Implement spectre mitigations part 1

* Implement machdep.spectre_mitigation.  This can be set as a tunable
  or sysctl'd later.  The tunable is only applicable if the BIOS has
  the appropriate microcode, otherwise you have to update the microcode
  first and then use sysctl to set the mode.

  This works similarly to Linux's IBRS support.

  mode 0 - Spectre IBPB MSRs disabled

  mode 1 - Sets IBPB MSR on USER->KERN transition and clear it
	   on KERN->USER.

  mode 2 - Leave IBPB set globally.  Do not toggle on USER->KERN or
	   KERN->USER transitions.

* Retest spectre microcode MSRs on microcode update.

* Spectre mode 1 is enabled by default if the microcode supports it.
  (we might change this to disabled by default, I'm still mulling it
  over).

* General performance effects (not counting the MMU separation mode,
  which is machdep.meltdown_mitigation and adds another 3% in overhead):

  Skylake loses around 5% for mode 1 and 12% for mode 2, verses mode 0.
  Haswell loses around 12% for mode 1 and 53% for mode 2, verses mode 0.

  Add another 3% if MMU separation is also turned on (aka
  machdep.meltdown_mitigation).

* General system call overhead effects on Skylake:

  machdep.meltdown_mitigation=0, machdep.spectre_mitigation=0	103ns
  machdep.meltdown_mitigation=1, machdep.spectre_mitigation=0	360ns
  machdep.meltdown_mitigation=1, machdep.spectre_mitigation=1	848ns
  machdep.meltdown_mitigation=1, machdep.spectre_mitigation=2	404ns

  Note that mode 1 has better overall performance for mixed user+kernel
  workloads despite having a much higher system call overhead, whereas
  mode 2 has lower system call overhead but generally lower overall
  performance because IBPB is enabled in usermode.