Strengthen the implementations of xShmMemoryBarrier on both the unix and

windows VFSes, so that they likely work even if SQLITE_THREADSAFE=0 is used.

FossilOrigin-Name: c6ab807b72ddfc1462f61aa91442b6fac04ace8a

src/os_unix.c

@@ -4660,7 +4660,8 @@ static void unixShmBarrier(
   sqlite3_file *fd                /* Database file holding the shared memory */
 ){
   UNUSED_PARAMETER(fd);
-  unixEnterMutex();
+  sqlite3MemoryBarrier();         /* compiler-defined memory barrier */
+  unixEnterMutex();               /* Also mutex, for redundancy */
   unixLeaveMutex();
 }