Avoid segfaults in 64-bit Windows builds

* src/lisp.h (COMMON_MULTIPLE): Move here from alloc.c.
* src/thread.c (THREAD_ALIGNMENT): New macro.
(main_thread): Use THREAD_ALIGNMENT to align propertly.  (Bug#29040)

src/alloc.c

@@ -621,12 +621,6 @@ buffer_memory_full (ptrdiff_t nbytes)
 #endif
 }
 
-/* A common multiple of the positive integers A and B.  Ideally this
-   would be the least common multiple, but there's no way to do that
-   as a constant expression in C, so do the best that we can easily do.  */
-#define COMMON_MULTIPLE(a, b) \
-  ((a) % (b) == 0 ? (a) : (b) % (a) == 0 ? (b) : (a) * (b))
-
 #ifndef XMALLOC_OVERRUN_CHECK
 #define XMALLOC_OVERRUN_CHECK_OVERHEAD 0
 #else

src/lisp.h

@@ -283,6 +283,12 @@ error !;
 # define GCALIGNED /* empty */
 #endif
 
+/* A common multiple of the positive integers A and B.  Ideally this
+   would be the least common multiple, but there's no way to do that
+   as a constant expression in C, so do the best that we can easily do.  */
+#define COMMON_MULTIPLE(a, b) \
+  ((a) % (b) == 0 ? (a) : (b) % (a) == 0 ? (b) : (a) * (b))
+
 /* Some operations are so commonly executed that they are implemented
    as macros, not functions, because otherwise runtime performance would
    suffer too much when compiling with GCC without optimization.

src/thread.c

@@ -26,7 +26,9 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 #include "coding.h"
 #include "syssignal.h"
 
-static struct thread_state alignas (GCALIGNMENT) main_thread;
+#define THREAD_ALIGNMENT COMMON_MULTIPLE (alignof (max_align_t), GCALIGNMENT)
+
+static struct thread_state alignas (THREAD_ALIGNMENT) main_thread;
 
 struct thread_state *current_thread = &main_thread;