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-openmp.sh

@@ -64,12 +64,26 @@ else
     esac
 fi
 
+ARM64X_FLAGS=""
+for arch in $ARCHS; do
+    case $arch in
+    arm64ec) ARM64X_FLAGS="-marm64x" ;;
+    esac
+done
+
 for arch in $ARCHS; do
     CMAKEFLAGS=""
+    FLAGS=""
     case $arch in
     x86_64)
         CMAKEFLAGS="$CMAKEFLAGS -DLIBOMP_ASMFLAGS=-m64"
         ;;
+    aarch64)
+        FLAGS="$ARM64X_FLAGS"
+        ;;
+    arm64ec)
+        continue
+        ;;
     esac
 
     [ -z "$CLEAN" ] || rm -rf build-openmp-$arch
@@ -90,8 +104,9 @@ for arch in $ARCHS; do
         -DCMAKE_RANLIB="$PREFIX/bin/llvm-ranlib" \
         -DLLVM_ENABLE_RUNTIMES="openmp" \
         -DLIBOMP_ENABLE_SHARED=TRUE \
-        -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 $FLAGS" \
+        -DCMAKE_SHARED_LINKER_FLAGS="$FLAGS" \
         $CMAKEFLAGS \
         ..
     cmake --build . ${CORES:+-j${CORES}}