x86emul: correct DECLARE_ALIGNED()

Stop creating an excessively large array on the stack, by properly
taking into account the array element size when establishing its
element count (and of course also when calculating the pointer to
be actually used to access the memory).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/x86_emulate/x86_emulate.c

@@ -553,10 +553,10 @@ typedef union {
  * the compiler for automatic variables. Use this helper to instantiate a
  * suitably aligned variable, producing a pointer to access it.
  */
-#define DECLARE_ALIGNED(type, var)                                   \
-    long __##var[sizeof(type) + __alignof(type) - __alignof(long)];  \
-    type *const var##p =                                             \
-        (void *)((long)(__##var + __alignof(type) - __alignof(long)) \
+#define DECLARE_ALIGNED(type, var)                                        \
+    long __##var[(sizeof(type) + __alignof(type)) / __alignof(long) - 1]; \
+    type *const var##p =                                                  \
+        (void *)(((long)__##var + __alignof(type) - __alignof(__##var))   \
                  & -__alignof(type))
 
 #ifdef __GCC_ASM_FLAG_OUTPUTS__