Define Turin Virtual Platforms

[rmustacc]

This covers both the CPUID related pieces and all of the surrounding virtual platform related features.

[iximeow]

PR: #9043

Aside from the EFER bits I don't think there's further Turin-specific virt stack work, though letting guests manage PRED_CMD and SPEC_CTRL is yet more valuable on Turin. It looks like all the opt hints are just true generally, for example. So when we get to https://www.illumos.org/issues/13435 and https://www.illumos.org/issues/13437 those will imply a new CPU platform rev for Milan and Turin both (if this lands before them).