Reinstate max_align_t alignment in mj_stackAllocByte.

PiperOrigin-RevId: 559433104
Change-Id: I891a6a0562cab1c3af0e3dc395304207ca3a3190

src/engine/engine_crossplatform.h

@@ -15,7 +15,15 @@
 #ifndef MUJOCO_SRC_ENGINE_ENGINE_CROSSPLATFORM_H_
 #define MUJOCO_SRC_ENGINE_ENGINE_CROSSPLATFORM_H_
 
-#include <stdlib.h>
+// IWYU pragma: begin_keep
+#if !defined(__cplusplus)
+  #include <stddef.h>
+  #include <stdlib.h>
+#else
+  #include <cstddef>
+  #include <cstdlib>
+#endif
+// IWYU pragma: end_keep
 
 // Windows
 #ifdef _WIN32
@@ -55,4 +63,8 @@
   #define mjFALLTHROUGH ((void) 0)
 #endif
 
+#if defined(_MSC_VER) && !defined(__clang__) && !defined(__cplusplus)
+  typedef long double max_align_t;
+#endif
+
 #endif  // MUJOCO_SRC_ENGINE_ENGINE_CROSSPLATFORM_H_