[llvm] [BOLT] Add writing support for Linux kernel ORC (PR #80950)

[Wentao Zhang via llvm-commits]

whentojump wrote:

> Hi @whentojump, in theory, it shouldn't matter how the profile for the kernel was collected. At the moment though, we don't have instrumentation support, so you have to use sampling.
> 
> For now, not all patches are in the main branch. Originally, there will be a constraint on the kernel config and version than are supported by BOLT. Once I upstream the first set, I'll publish the instructions.

Thanks for the explanations! I look forward to the more complete work :))

> we don't have instrumentation support

I suppose one reason is on the kernel side we don't have a way of exporting the profile? If that's true, then we are in the same stage as [PGO](https://lore.kernel.org/linux-doc/20210407211704.367039-1-morbo@google.com/). 

https://github.com/llvm/llvm-project/pull/80950