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-unix.sh

@@ -51,6 +51,10 @@ 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