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.