- Special-case NMI handling on the AMD64.

  On entry or exit from the kernel the 'alltraps' and 'doreti' code
  used taken by normal traps disables interrupts to protect the
  critical sections where it is setting up %gs.

  This protection is insufficient in the presence of NMIs since NMIs
  can be taken even when the processor has disabled normal interrupts.
  Thus the NMI handler needs to actually read MSR_GBASE on entry to
  the kernel to determine whether a swap of %gs using 'swapgs' is
  needed.  However, reads of MSRs are expensive and integrating this
  check into the 'alltraps'/'doreti' path would penalize normal
  interrupts.

- Teach DDB about the 'nmi_calltrap' symbol.

Reviewed by:	bde, peter (older versions of this change)

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