Create the prefix/triplet/bin directory before installing DLLs in build-libcxx.sh

If the triplet directory is wiped and only the build-mingw-w64.sh
step is rerun, the bin subdirectory would be missing, ending up
with a file named "bin", which actually was the libc++ DLL.

build-libcxx.sh

@@ -111,6 +111,7 @@ build_all() {
         make -j$CORES
         make install
         if [ "$type" = "shared" ]; then
+            mkdir -p $PREFIX/$arch-w64-mingw32/bin
             cp lib/libunwind.dll $PREFIX/$arch-w64-mingw32/bin
         else
             # Merge libpsapi.a into the static library libunwind.a, to