[llvm] [compiler-rt] Pass the CMake binary dir directly (PR #102493)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> What about `function(runtime_default_target)` and `function(runtime_register_target name)`, do they not need similar changes? (Sorry, didn't read the whole discussion.)

I don't think so, the issue was that `compiler-rt` used a really confounding name for `LLVM_CMAKE_DIR` which conflicted with LLVM's `LLVM_CMAKE_DIR`. The regular runtime targets don't use this AFAIK.

https://github.com/llvm/llvm-project/pull/102493