LLVM doesn't build on the first try when cross compilling to windows-gnu host

[mati865]

Running ./x.py build --host x86_64-pc-windows-gnu --target x86_64-pc-windows-gnu from Linux will fail on building LLVM for Windows (so it's 2nd LLVM build) because somehow LLVM_ON_UNIX is defined:

/home/mateusz/Projects/rust/rust/src/llvm-project/llvm/include/llvm/Support/FileSystem.h:164:3: error: ‘uid_t’ does not name a type; did you mean ‘pid_t’?
  164 |   uid_t fs_st_uid = 0;
      |   ^~~~~
      |   pid_t

Magic happens when running the same command second time without cleaning build dir or changing anything. It will finish LLVM build and install the files.

Rust build will fail on librustc_codegen_llvm because of missing shell32 but this could be unrelated (although CMake should link it).