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-libcxx.sh

@@ -72,6 +72,12 @@ else
     esac
 fi
 
+for arch in $ARCHS; do
+    case $arch in
+    arm64ec) ARM64X_FLAGS="-marm64x" ;;
+    esac
+done
+
 for arch in $ARCHS; do
     [ -z "$CLEAN" ] || rm -rf build-$arch
     mkdir -p build-$arch
@@ -79,12 +85,20 @@ for arch in $ARCHS; do
     [ -n "$NO_RECONF" ] || rm -rf CMake*
 
     EXTRA_CFLAGS=""
+    EXTRA_LDFLAGS=""
     case $arch in
     i686|x86_64)
         # Force using the mingw stdio functions, for correct long double
         # printing.
         EXTRA_CFLAGS="-D__USE_MINGW_ANSI_STDIO=1"
         ;;
+    aarch64)
+        EXTRA_CFLAGS="$ARM64X_FLAGS"
+        EXTRA_LDFLAGS="$ARM64X_FLAGS"
+        ;;
+    arm64ec)
+        continue
+        ;;
     esac
 
     cmake \
@@ -119,6 +133,8 @@ for arch in $ARCHS; do
         -DLIBCXXABI_LIBDIR_SUFFIX="" \
         -DCMAKE_C_FLAGS_INIT="$CFGUARD_CFLAGS $EXTRA_CFLAGS" \
         -DCMAKE_CXX_FLAGS_INIT="$CFGUARD_CFLAGS $EXTRA_CFLAGS" \
+        -DCMAKE_ASM_FLAGS_INIT="$EXTRA_CFLAGS" \
+        -DCMAKE_SHARED_LINKER_FLAGS="$EXTRA_LDFLAGS" \
         ..
 
     cmake --build . ${CORES:+-j${CORES}}