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

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> @jhuber6 the reason I initially implemented builtins and runtimes separately is because the runtime configuration completing successfully and accurately depends on the builtin libraries being available during configuration. We could probably make some educated guess hard-coded assumptions about what the toolchain supports, but you can quickly get off the rails with more exotic target support.

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.

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