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

[Vincent Lee via llvm-commits]

thevinster wrote:

> Does this variable need to be cached?

Yes, otherwise LLVM will not pick up these values. I'll add it to the documentation. Thanks for the pointer. 

> 1.4x as fast as the baseline or the new speed is 240% of the baseline?

1.4x as fast as the baseline. So, as an example, if an overall build took 1 hour to build everything with LTO, it would've taken us around ~43 minutes to build our toolchains with LTO because we selectively only chose what we wanted to optimize. YMMV depending on the machine instance and what you decide to not optimize. 

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