[llvm] [offload] Standalone build fixes (PR #118173)

[Michał Górny via llvm-commits]

mgorny wrote:

> I personally would appreciate to rebase this PR and add the changes required to work with the libc patch. Don't know how difficult that will be though.

Actually, the right approach would be to fix the libc path not to use `LLVM_MAIN_SRC_DIR` (after all, it's not using `llvm` subdirectory at all), and instead declare a path relative to `CMAKE_CURRENT_SOURCE_DIR`, much like `LLVM_COMMON_CMAKE_UTILS` is declared now.

https://github.com/llvm/llvm-project/pull/118173