[cmake] Pass /winsysroot explicitly

[smeenai]

Summary

We were relying on HandleLLVMOptions adding this flag, but that's not
included in compiler-rt. Add the flag explicitly ourselves to ensure
it's present; the duplication of the flag in the parts of the build
which do include HandleLLVMOptions doesn't cause any issues.

[smeenai]

Alphabetize the flags

[ychen]

LGTM. Thanks.

llvm/cmake/platforms/WinMsvc.cmake

 # for the target, makes sure these are unset during build time.
 set(CROSS_TOOLCHAIN_FLAGS_NATIVE "${_CTF_NATIVE_DEFAULT}" CACHE STRING "")
 
 set(COMPILE_FLAGS
     -D_CRT_SECURE_NO_WARNINGS
     --target=${TRIPLE_ARCH}-windows-msvc
     -fms-compatibility-version=19.20
     -vctoolsversion ${MSVC_VER}
-    -winsdkversion ${WINSDK_VER})
+    -winsdkversion ${WINSDK_VER}
+    -winsysroot ${LLVM_WINSYSROOT})
 
 if(case_sensitive_filesystem)
   # Ensure all sub-configures use the top-level VFS overlay instead of generating their own.
   init_user_prop(winsdk_vfs_overlay_path)
   if(NOT winsdk_vfs_overlay_path)
     set(winsdk_vfs_overlay_path "${CMAKE_BINARY_DIR}/winsdk_vfs_overlay.yaml")
     generate_winsdk_vfs_overlay("${WINSDK_INCLUDE}" "${winsdk_vfs_overlay_path}")
     init_user_prop(winsdk_vfs_overlay_path)