Stop manually setting CMAKE_CROSSCOMPILING

This is set implicitly when CMAKE_SYSTEM_NAME is set. According to
the docs, the user isn't supposed to set it manually.

build-llvm.sh

@@ -148,7 +148,6 @@ CMAKEFLAGS="$LLVM_CMAKEFLAGS"
 
 if [ -n "$HOST" ]; then
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_SYSTEM_NAME=Windows"
-    CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_CROSSCOMPILING=TRUE"
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_C_COMPILER=$HOST-gcc"
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_CXX_COMPILER=$HOST-g++"
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_RC_COMPILER=$HOST-windres"