build-libcxx: Install the libc++ module in a shared directory

By default these files would be installed into
${CMAKE_INSTALL_PREFIX}/share/libc++/v1, where
${CMAKE_INSTALL_PREFIX} is equal to ${PREFIX}/${arch}-w64-mingw32.

However, to avoid duplicating these files for each architecture,
install them into ${PREFIX}/share/libc++/v1 instead.

While the files aren't very large (currently weighing in at around
612 KB), they're architecture independent files (and they're
more than a few; they're currently 135 files), so we should
share them if possible.

build-libcxx.sh

@@ -104,6 +104,7 @@ for arch in $ARCHS; do
         -DLIBCXX_LIBDIR_SUFFIX="" \
         -DLIBCXX_INCLUDE_TESTS=FALSE \
         -DLIBCXX_INSTALL_MODULES=ON \
+        -DLIBCXX_INSTALL_MODULES_DIR="$PREFIX/share/libc++/v1" \
         -DLIBCXX_ENABLE_ABI_LINKER_SCRIPT=FALSE \
         -DLIBCXXABI_USE_COMPILER_RT=ON \
         -DLIBCXXABI_USE_LLVM_UNWINDER=ON \