x86: Expose the raw hash map of MSRs

This patch allows the KVM CPU module to initialize it's MSRs by
enumerating the MSRs in the gem5 x86 implementation.

X86: Move the MSR lookup table out of the TLB and into its own file.

Translating MSR addresses into MSR register indices took a lot of space in the
TLB source and made looking around in that file awkward. This change moves
the lookup into its own file to get it out of the way. It also changes it from
a switch statement to a hash map which should hopefully be a little more
efficient.