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).

build-mingw-w64.sh

@@ -105,6 +105,10 @@ $MAKE install
 cd ../..
 if [ -z "$SKIP_INCLUDE_TRIPLET_PREFIX" ]; then
     for arch in $ARCHS; do
+        case $arch in
+        arm64ec) continue ;;
+        esac
+
         mkdir -p "$PREFIX/$arch-w64-mingw32"
         if [ ! -e "$PREFIX/$arch-w64-mingw32/include" ]; then
             ln -sfn ../generic-w64-mingw32/include "$PREFIX/$arch-w64-mingw32/include"
@@ -112,17 +116,24 @@ if [ -z "$SKIP_INCLUDE_TRIPLET_PREFIX" ]; then
     done
 fi
 
+ARM64X_FLAGS=""
+for arch in $ARCHS; do
+    case $arch in
+    arm64ec) ARM64X_FLAGS="--enable-arm64x" ;;
+    esac
+done
+
 cd mingw-w64-crt
 for arch in $ARCHS; do
-    [ -z "$CLEAN" ] || rm -rf build-$arch
-    mkdir -p build-$arch
-    cd build-$arch
     case $arch in
     armv7)
         FLAGS="--disable-lib32 --disable-lib64 --enable-libarm32"
         ;;
-    aarch64|arm64ec)
-        FLAGS="--disable-lib32 --disable-lib64 --enable-libarm64"
+    aarch64)
+        FLAGS="--disable-lib32 --disable-lib64 --enable-libarm64 $ARM64X_FLAGS"
+        ;;
+    arm64ec)
+        continue
         ;;
     i686)
         FLAGS="--enable-lib32 --disable-lib64"
@@ -133,6 +144,9 @@ for arch in $ARCHS; do
     esac
     FLAGS="$FLAGS --with-default-msvcrt=$DEFAULT_MSVCRT"
     FLAGS="$FLAGS --enable-silent-rules"
+    [ -z "$CLEAN" ] || rm -rf build-$arch
+    mkdir -p build-$arch
+    cd build-$arch
     ../configure --host=$arch-w64-mingw32 --prefix="$PREFIX/$arch-w64-mingw32" $FLAGS $CFGUARD_FLAGS $CRT_CONFIG_FLAGS
     $MAKE -j$CORES
     $MAKE install
@@ -141,6 +155,10 @@ done
 cd ..
 
 for arch in $ARCHS; do
+    case $arch in
+    arm64ec) continue ;;
+    esac
+
     if [ ! -f $PREFIX/$arch-w64-mingw32/lib/libssp.a ]; then
         # Create empty dummy archives, to avoid failing when the compiler
         # driver adds "-lssp -lssh_nonshared" when linking.