[CMAKE] Fix compiling 32-bit targets on 64-bit clang-cl

With help of @tkreuzer and @julenuri I got x86_64-pc-windows-msvc flavor
of LLVM 13.0.1 compiling 32-bit ReactOS.

Author: SigmaTel71

toolchain-msvc.cmake

@@ -39,6 +39,8 @@ if(USE_CLANG_CL)
         set(CMAKE_C_COMPILER_TARGET "arm64-pc-windows-msvc")
         set(CMAKE_CXX_COMPILER_TARGET "arm64-pc-windows-msvc")
     else()
+        set(CMAKE_C_FLAGS "-m32")
+        set(CMAKE_CXX_FLAGS "-m32")
         set(CMAKE_C_COMPILER_TARGET "i686-pc-windows-msvc")
         set(CMAKE_CXX_COMPILER_TARGET "i686-pc-windows-msvc")
     endif()