Fix cross-compilation of LLVM to aarch64 Windows targets

Fix cross-compilation of LLVM to aarch64 Windows targets

When cross-compiling, the LLVM build system recurses to build tools
that need to run on the host system. However, since we pass cmake defines
to set the compiler and target, LLVM still compiles these tools for the
target system, rather than the host. The tools then fail to execute
during the LLVM build.

This change sets defines for the tools that need to run on the
host (llvm-nm, llvm-tablegen, and llvm-config), so that the LLVM build
does not attempt to build them, and instead relies on the tools already built.

If compiling with clang-cl, this change also adds the `--target` option
to specify the target triple. MSVC compilers do not require this, since there
is a separate compiler binary for cross-compilation.