kdb: add rdmsr and wrmsr commands for i386

[Bernardo Innocenti]

Keith Owens wrote:

> Before using MSR, you must first check that the cpu supports the
> instruction, rd/wrmsr cause an oops on 486 or earlier.  Also using an
> invalid msr number causes an oops, so use rd/wrmsr_safe().

I didn't bother implementing those checks because kdb recovers
nicely from GPF anyway.  It's the valid MSR writes that could
cause unrecoveable problems! :)

-- 
   // Bernardo Innocenti
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