[llvm] OrcV2: also set COFF flag overrides when custom linking layer is used (PR #129533)

[Lang Hames via llvm-commits]

lhames wrote:

> > What customizations are you using? Do you need to control layout, or just find out where particular sections / symbols are?
> 
> I need to control the layout. I am using `LLVMOrcCreateRTDyldObjectLinkingLayerWithMCJITMemoryManagerLikeCallbacks` to place generated code into a flat buffer that can contains everything (code, data). The program only uses %rip-relative accesses. An application can later map this data into memory and run it without using any of the LLVM infrastructure.
> 
> These are entirely self-contained computational kernels that don't use any external functionality (e.g. from libc).

This will be interesting. The `JITLink` and `JITLinkMemoryManager` APIs are much more complex than `RuntimeDyld`. I don't think we can expose the raw C API for this.

Do you actually need to control where the JIT'd memory lives, or just its layout? I'm wondering if we could get by with a "SectionMerger" plugin that merges all blocks into a single section with `RWX` permissions, plus maybe a custom section-based memory manager if you need to control where it goes?

https://github.com/llvm/llvm-project/pull/129533