[x86_64] Add LFENCE after SWAPGS
|Reported by:||waddlesplash||Owned by:||waddlesplash|
|Has a Patch:||no||Platform:||x86-64|
This is to patch yet another CPU vulerability: https://arstechnica.com/information-technology/2019/08/silent-windows-update-patched-side-channel-that-leaked-data-from-intel-cpus/
This looks like the simplest Intel mitigation yet, so there's no reason we can't apply it.