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-unix.sh

@@ -51,10 +51,6 @@ rm -rf $DEST/lib/clang/*/lib/linux
 
 # Copy all arch-specific subdirectories plus the "generic" one, as is.
 for arch in generic $ARCHS; do
-    case $arch in
-    arm64ec) continue ;;
-    esac
-
     rm -rf $DEST/$arch-w64-mingw32
     cp -a $SRC/$arch-w64-mingw32 $DEST/$arch-w64-mingw32
 done