[llvm] [LLVM][runtimes] Prepopulate `LLVM_BUILTIN_TARGETS` with runtimes values (PR #95554)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> > I see, so it's related to the CMake configure step when it tries to identify features? I can see that being an issue for really specific checks where the compiler defaults to compiler-rt for its runtime instead of some GNU libs. Still unfortunate that it adds such a large overhead to build times.
> 
> To make the point, who has GNU libs at all? I don't in 3 of the 4 configurations I run LLVM's build for.
> 
> Yes it is unfortunate, but there are more build configurations than we can reasonably hard code predictions for.

Alright, good to know the motivation behind it. In any case, this patch should fix it to where users will have the builtins built without needing to manually specify an extra variable.

https://github.com/llvm/llvm-project/pull/95554