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-openmp.sh

@@ -80,7 +80,6 @@ for arch in $ARCHS; do
         -DCMAKE_CXX_COMPILER=$arch-w64-mingw32-clang++ \
         -DCMAKE_RC_COMPILER=$arch-w64-mingw32-windres \
         -DCMAKE_ASM_MASM_COMPILER=llvm-ml \
-        -DCMAKE_CROSSCOMPILING=TRUE \
         -DCMAKE_SYSTEM_NAME=Windows \
         -DCMAKE_AR="$PREFIX/bin/llvm-ar" \
         -DCMAKE_RANLIB="$PREFIX/bin/llvm-ranlib" \