[llvm] [BOLT] Add CustomOffset flag for optimizing kernel (PR #98153)

[via llvm-commits]

lifengxiang1025 wrote:

After reading BOLT's OptimizingLinux.md last week, we two guys decided to try this. And then we met the problem that `perf2bolt` output empty `.fdata` file. It tooks us about two days  to fix it and find the reason. 
Our main aim is not merging this PR. We want someone else who met same problem don't need to take time to investigate.
I can think out two ways to resolve this problem: 
1. Add all address by one offset after perf2bolt parse it when optimizing kernel.
2. Add one tip in OptimizingLinux.md to suggest people deactivating KASLR.
Maybe there are more graceful ways, can you give us some suggestion? 

https://github.com/llvm/llvm-project/pull/98153