Meltdown/Spectre mitigation for 4.15 and beyond

> One other issue with retpolines remains somewhat unresolved, though: using them requires support from the compiler, and almost nobody has a compiler with that support available.

FreeBSD's decision from years back to migrate to the llvm toolchain for its compiler (and eventually linker on amd64) gives it this support. In HardenedBSD, we've already switched amd64 to use ld.lld as the default linker. We're testing a full OS (world + kernel + packages) with retpoline right now. We'll likely debut retpoline in HardenedBSD 12-CURRENT/amd64 late this week or early next.