Add ARM64X support

Extend the aarch64-w64-mingw32 prefix to provide ARM64EC libraries
alongside standard AArch64 libraries, replacing the separate
arm64ec-w64-mingw32 prefix. This aligns with how MSVC distributes
ARM64EC libraries and enables the linker to process both targets
simultaneously into a single hybrid binary.

This extensively utilizes LLVM's -marm64x option, compiling both
targets in a single compiler invocation to produce hybrid object
files (unlike MSVC tools, which require separate compilation passes).

prepare-cross-toolchain.sh

@@ -61,6 +61,10 @@ 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