Use -unwindlib=libunwind instead of merging libunwind into libc++

This makes the build of libcxx a bit cleaner, and simplifies later
building and testing of libcxx.

build-libcxx.sh

@@ -193,15 +193,6 @@ build_all() {
         cd build-$arch-$type
         $BUILDCMD ${CORES+-j$CORES}
         $BUILDCMD install
-        if [ "$type" = "shared" ]; then
-            llvm-ar qcsL \
-                "$PREFIX/$arch-w64-mingw32/lib/libc++.dll.a" \
-                "$PREFIX/$arch-w64-mingw32/lib/libunwind.dll.a"
-        else
-            llvm-ar qcsL \
-                "$PREFIX/$arch-w64-mingw32/lib/libc++.a" \
-                "$PREFIX/$arch-w64-mingw32/lib/libunwind.a"
-        fi
         cd ..
     done
     cd ..
@@ -211,3 +202,13 @@ build_all() {
 # work when linking against the DLL, but not vice versa.
 [ -z "$BUILD_SHARED" ] || build_all shared
 [ -z "$BUILD_STATIC" ] || build_all static
+
+# Remove dummy placeholder libunwind.a. If we've built a static version, it
+# already was overwritten with a proper one, but if only building a shared
+# version, remove the dummy one to avoid surprises.
+for arch in $ARCHS; do
+    # Only remove the file if it contains no members.
+    if [ -f $PREFIX/$arch-w64-mingw32/lib/libunwind.a ] && [ "$(llvm-ar t $PREFIX/$arch-w64-mingw32/lib/libunwind.a)" = "" ]; then
+        rm $PREFIX/$arch-w64-mingw32/lib/libunwind.a
+    fi
+done