Profile-guided optimization for the kernel

We've reached out to Ian about collaborating on LTO integration; we're in the process of upstreaming LTO patches for LLVM and need help from the GCC side to test and sort out what issues they might run into: https://lore.kernel.org/lkml/CAKwvOdkbkvXdfXLzTNOj8m8_YWj...

For example, LTO has exposed a handful of compiler bugs. Most recently: https://bugs.llvm.org/show_bug.cgi?id=47479.