Mitigate L1TF (Intel processor vulnerability)

[waddlesplash]

This is actually both a worse vulnerability than Meltdown, and easier to mitigate, requiring only a change in how we set flags on page tables: ​https://blogs.technet.microsoft.com/srd/2018/08/14/analysis-and-mitigation-of-l1-terminal-fault-l1tf/

Actually it may already be "mitigated", I just don't know enough to investigate properly.

[waddlesplash]

The linked post indicates that L1TF can be mitigated by:

1) the physical page referred to by the page frame bits of not present page table entries always contain benign data and/or 2) a high order bit is set in the page frame bits that does not correspond to accessible physical memory.

It appears (1) isn't the case already because we just use ClearTableEntryFlags and not ClearTableEntry in Unmap() in the x86 translation maps.

[waddlesplash]

Adjustment made in hrev58662, but it only affected kernel maps anyway. I don't think there's anything else that need to be done here.