Add a terminator to the domain_state

On a 32-bits architecture, if the last member of the caml_domain_state
is a pointer, it will be 4-bytes long, and the struct will end after a
4 bytes boundary, failing the assertion.

As @xavierleroy [put it][1]:

> we don't care about the total size of the struct, but we care that
> the members before `end_of_domain_state` are spaced by 8.

[1]: https://github.com/ocaml/ocaml/issues/9714#issuecomment-650516382

See #9714.

runtime/caml/domain_state.h

@@ -33,6 +33,7 @@ typedef struct {
 #endif
 #include "domain_state.tbl"
 #undef DOMAIN_STATE
+    CAMLalign(8) char end_of_domain_state;
 } caml_domain_state;
 
 enum {
@@ -45,9 +46,8 @@ enum {
 /* Check that the structure was laid out without padding,
    since the runtime assumes this in computing offsets */
 CAML_STATIC_ASSERT(
-  sizeof(caml_domain_state) ==
-   (Domain_state_num_fields
-   ) * 8);
+    offsetof(caml_domain_state, end_of_domain_state) ==
+    Domain_state_num_fields * 8);
 
 CAMLextern caml_domain_state* Caml_state;
 #ifdef CAML_NAME_SPACE