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-compiler-rt.sh

@@ -134,7 +134,26 @@ if [ -n "$NATIVE" ]; then
     exit 0
 fi
 
+ARM64X_FLAGS=""
+if [ -z "$SANITIZERS" ]; then
+    for arch in $ARCHS; do
+        case $arch in
+        arm64ec) ARM64X_FLAGS="-marm64x" ;;
+        esac
+    done
+fi
+
 for arch in $ARCHS; do
+    FLAGS=""
+    case $arch in
+    aarch64)
+        FLAGS="$ARM64X_FLAGS"
+        ;;
+    arm64ec)
+        continue
+        ;;
+    esac
+
     [ -z "$CLEAN" ] || rm -rf build-$arch$BUILD_SUFFIX
     mkdir -p build-$arch$BUILD_SUFFIX
     cd build-$arch$BUILD_SUFFIX
@@ -160,17 +179,12 @@ for arch in $ARCHS; do
         -DCMAKE_FIND_ROOT_PATH_MODE_INCLUDE=ONLY \
         -DCMAKE_FIND_ROOT_PATH_MODE_PACKAGE=ONLY \
         -DSANITIZER_CXX_ABI=libc++ \
-        -DCMAKE_C_FLAGS_INIT="$CFGUARD_CFLAGS" \
-        -DCMAKE_CXX_FLAGS_INIT="$CFGUARD_CFLAGS" \
+        -DCMAKE_C_FLAGS_INIT="$CFGUARD_CFLAGS $FLAGS" \
+        -DCMAKE_CXX_FLAGS_INIT="$CFGUARD_CFLAGS $FLAGS" \
+        -DCMAKE_ASM_FLAGS_INIT="$FLAGS" \
         $SRC_DIR
     cmake --build . ${CORES:+-j${CORES}}
 
-    # Skip install on arm64ec, we merge archives instead.
-    if [ "$arch" = "arm64ec" ]; then
-        cd ..
-        continue
-    fi
-
     cmake --install . --prefix "$INSTALL_PREFIX"
     mkdir -p "$PREFIX/$arch-w64-mingw32/bin"
     if [ -n "$SANITIZERS" ]; then
@@ -198,20 +212,6 @@ for arch in $ARCHS; do
     cd ..
 done
 
-# Clang expects the aarch64 compiler-rt name on ARM64EC. While this could be adjusted
-# in Clang, the current approach mirrors MSVC, where the core CRT is provided as
-# archives containing both EC and native support. Ideally, the LLVM build system would
-# handle this automatically, but for now we can merge it here.
-for arch in $ARCHS; do
-    if [ "$arch" = "arm64ec" ]; then
-        rm -f "$INSTALL_PREFIX/lib/windows/libclang_rt.builtins-aarch64.a" \
-              "$INSTALL_PREFIX/lib/windows/libclang_rt.builtins-arm64ec.a"
-        "$PREFIX/bin/llvm-lib" -machine:arm64ec "-out:$INSTALL_PREFIX/lib/windows/libclang_rt.builtins-aarch64.a" \
-                               build-aarch64/lib/windows/libclang_rt.builtins-aarch64.a \
-                               build-arm64ec/lib/windows/libclang_rt.builtins-arm64ec.a
-    fi
-done
-
 if [ "$INSTALL_PREFIX" != "$CLANG_RESOURCE_DIR" ]; then
     # symlink to system headers - skip copy
     rm -rf "$INSTALL_PREFIX/include"