x86: Support capability-wide MSRs

[bsdjhb]

In place of the the current capability control registers in version 9, CHERI-x86 should selectively extend certain MSRs to hold capabilities. This would require new variants of RDMSR and WRMSR (presumably named RDMSRX and WRMSRX) that would store the MSR value in CAX rather than EAX:EDX. This would be useful even for 64-bit MSRs which would use a NULL-derived MSR value in CAX.

This provides a natural base on which to describe CHERI extensions for FRED.