Auto merge of #98328 - topjohnwu:fix_cross, r=jyn514

Auto merge of #98328 - topjohnwu:fix_cross, r=jyn514

Fix several issues during cross compiling

- When cross compiling LLVM on an arm64 macOS machine to x86_64, CMake will produce universal binaries by default, causing link errors. Explicitly set `CMAKE_OSX_ARCHITECTURES` to the one single target architecture so that the executables and libraries will be single architecture.
- When cross compiling rustc with `llvm.clang = true`, `CLANG_TABLEGEN` has to be set to the host `clang-tblgen` executable to build clang.