Prepare the opt-in Windows cross C/C++ toolchain for Linux execution from Windows clients: pin a minimal LLVM archive, stage SDK/library case aliases and a single Clang resource directory, generate host wrappers, and support Linux execution-tool repository overrides and dependency-path aliases.

Use `MONGO_WINDOWS_CROSS_EXEC_ARCH` consistently for LLVM selection and Linux execution constraints, and pass the Windows wrapper's actual pool as a tracked repo input. Windows cross toolchains require x86_64 Linux execution on both Windows and Linux clients. Both C++ and GDB repositories track the shared toolchain overrides.

Extracted from #63670 as an independent PR targeting master. Persistent-container runner wiring, Rust/WASI integration, resource compilation rules, and the default/CI rollout follow separately.