[llvm] [CMake] Support per-target linker flags (PR #68393)

[Petr Hosek via llvm-commits]

petrhosek wrote:

Reducing the overhead of LTO was one of the motivations for `-ffat-lto-objects, which let you use LTO for distribution components while disabling LTO for everything else; @ilovepi is working on a patch for that which should be ready for review soon.

This change might be still useful if you want to enable/disable LTO only for certain targets. My concern is the granularity, specifically how practical is controlling linker flags at the per-target level, especially if you build a large number of targets? The alternatives I could think of is setting the linker flags at the per-distribution granularity.

https://github.com/llvm/llvm-project/pull/68393