[mlir] Fix building CRunnerUtils on OpenBSD with 15.x

CRunnerUtils builds as C++11. 9c1d133 broke
the build on OpenBSD. aligned_alloc() was only introduced in C++17.

mlir/lib/ExecutionEngine/CRunnerUtils.cpp

@@ -127,14 +127,10 @@ extern "C" void *_mlir_alloc(uint64_t size) { return malloc(size); }
 extern "C" void *_mlir_aligned_alloc(uint64_t alignment, uint64_t size) {
 #ifdef _WIN32
   return _aligned_malloc(size, alignment);
-#elif defined(__APPLE__)
-  // aligned_alloc was added in MacOS 10.15. Fall back to posix_memalign to also
-  // support older versions.
+#else
   void *result = nullptr;
   (void)::posix_memalign(&result, alignment, size);
   return result;
-#else
-  return aligned_alloc(alignment, size);
 #endif
 }