[mlir] Workaround for export lib generation on Windows for mlir_arm_sme_abi_stubs

Using mlir smake in downstream project faild with error
```
CMake Error at D:/projs/llvm/llvm-install/lib/cmake/mlir/MLIRTargets.cmake:2537 (message):
  The imported target "mlir_arm_sme_abi_stubs" references the file

     "D:/projs/llvm/llvm-install/lib/mlir_arm_sme_abi_stubs.lib"

  but this file does not exist.  Possible reasons include:

  * The file was deleted, renamed, or moved to another location.

  * An install or uninstall procedure did not complete successfully.

  * The installation package was faulty and contained

     "D:/projs/llvm/llvm-install/lib/cmake/mlir/MLIRTargets.cmake"

  but not all the files it references.

Call Stack (most recent call first):
  D:/projs/llvm/llvm-install/lib/cmake/mlir/MLIRConfig.cmake:37 (include)
  mlir/CMakeLists.txt:5 (find_package)
```

Windows cmake needs export libaries but it seems they are only being generated if you have at least one exported symbol.
Add a dummy symbol to lib (export macros is copied from other mlir runnner libs).

Author: Hardcode84

mlir/lib/ExecutionEngine/ArmSMEStubs.cpp

@@ -10,6 +10,20 @@
 #include <cstdint>
 #include <iostream>
 
+#ifdef _WIN32
+#ifndef MLIR_RUNNERUTILS_EXPORT
+#ifdef mlir_arm_sme_abi_stubs_EXPORTS
+// We are building this library
+#define MLIR_ARNSMEABISTUBS_EXPORT __declspec(dllexport)
+#else
+// We are using this library
+#define MLIR_ARNSMEABISTUBS_EXPORT __declspec(dllimport)
+#endif // mlir_runner_utils_EXPORTS
+#endif // MLIR_RUNNERUTILS_EXPORT
+#else
+#define MLIR_ARNSMEABISTUBS_EXPORT LLVM_ATTRIBUTE_WEAK
+#endif // _WIN32
+
 // The actual implementation of these routines is in:
 // compiler-rt/lib/builtins/aarch64/sme-abi.S. These stubs allow the current
 // ArmSME tests to run without depending on compiler-rt. This works as we don't
@@ -19,7 +33,7 @@
 
 extern "C" {
 
-bool LLVM_ATTRIBUTE_WEAK __aarch64_sme_accessible() {
+bool MLIR_ARNSMEABISTUBS_EXPORT __aarch64_sme_accessible() {
   // The ArmSME tests are run within an emulator so we assume SME is available.
   return true;
 }
@@ -29,20 +43,20 @@ struct sme_state {
   int64_t x1;
 };
 
-sme_state LLVM_ATTRIBUTE_WEAK __arm_sme_state() {
+sme_state MLIR_ARNSMEABISTUBS_EXPORT __arm_sme_state() {
   std::cerr << "[warning] __arm_sme_state() stubbed!\n";
   return sme_state{};
 }
 
-void LLVM_ATTRIBUTE_WEAK __arm_tpidr2_restore() {
+void MLIR_ARNSMEABISTUBS_EXPORT __arm_tpidr2_restore() {
   std::cerr << "[warning] __arm_tpidr2_restore() stubbed!\n";
 }
 
-void LLVM_ATTRIBUTE_WEAK __arm_tpidr2_save() {
+void MLIR_ARNSMEABISTUBS_EXPORT __arm_tpidr2_save() {
   std::cerr << "[warning] __arm_tpidr2_save() stubbed!\n";
 }
 
-void LLVM_ATTRIBUTE_WEAK __arm_za_disable() {
+void MLIR_ARNSMEABISTUBS_EXPORT __arm_za_disable() {
   std::cerr << "[warning] __arm_za_disable() stubbed!\n";
 }
 }

mlir/lib/ExecutionEngine/CMakeLists.txt

@@ -181,6 +181,7 @@ if(LLVM_ENABLE_PIC)
   add_mlir_library(mlir_arm_sme_abi_stubs
     SHARED
     ArmSMEStubs.cpp)
+  target_compile_definitions(mlir_arm_sme_abi_stubs PRIVATE mlir_arm_sme_abi_stubs_EXPORTS)
 
   if(MLIR_ENABLE_CUDA_RUNNER)
     # Configure CUDA support. Using check_language first allows us to give a