Revert "Add ARM64X support"

This reverts commit 64effa9a71.

It turns out that the LLD handling of the TLS data structures
("_tls_used" and "_tls_index") needs more work, causing the
arm64ec side of arm64x binaries to not get them hooked up properly.

prepare-cross-toolchain.sh

@@ -61,10 +61,6 @@ mkdir -p $DEST/include
 # individual architectures.
 cp -a $SRC/generic-w64-mingw32/include/. $DEST/include
 for arch in $ARCHS; do
-    case $arch in
-    arm64ec) continue ;;
-    esac
-
     mkdir -p $DEST/$arch-w64-mingw32
     for subdir in bin lib share; do
         cp -a $SRC/$arch-w64-mingw32/$subdir $DEST/$arch-w64-mingw32