Remove the workaround with the initial, dummy empty libunwind.a

This reverts parts of commit e35ef3b7d7.

This is now handled better upstream, since
7af584ed87cc6eddb6adbc451c90fb8867469e06 in llvm-project
(November 5th 2021).

build-compiler-rt.sh

@@ -100,22 +100,7 @@ for arch in $ARCHS; do
     $BUILDCMD ${CORES+-j$CORES}
     $BUILDCMD install
     mkdir -p "$PREFIX/$arch-w64-mingw32/bin"
-    if [ -z "$SANITIZERS" ]; then
-        # Building builtins only. If libunwind isn't built yet, linking of any
-        # executable will still fail as we're using -unwindlib=libunwind.
-        # Therefore, create a dummy libunwind.a (unless there's a real
-        # libunwind installed already) to let linking succeed.
-        #
-        # This is a workaround to avoid needing to specify -unwindlib=none
-        # for all linking until libunwind has been built. This avoids
-        # needing to build libunwind right away (allowing building plain C
-        # executables already now) and avoids needing to pass -unwindlib=none
-        # when configuring the libunwind build.
-        if [ ! -f $PREFIX/$arch-w64-mingw32/lib/libunwind.a ] && [ ! -f $PREFIX/$arch-w64-mingw32/lib/libunwind.dll.a ]; then
-            # Create an archive, don't add any members.
-            llvm-ar rcs $PREFIX/$arch-w64-mingw32/lib/libunwind.a
-        fi
-    else
+    if [ -n "$SANITIZERS" ]; then
         mv "$PREFIX/lib/clang/$CLANG_VERSION/lib/windows/"*.dll "$PREFIX/$arch-w64-mingw32/bin"
     fi
     cd ..