ioq: Ensure ioq_ent is sufficiently aligned

The natural alignment of struct ioq_ent is only 2 on m68k, so over-align
it to at least 4 bytes on all platforms.

Link: https://buildd.debian.org/status/fetch.php?pkg=bfs&arch=m68k&ver=3.1-1&stamp=1707699583

src/ioq.c

@@ -177,7 +177,7 @@ typedef atomic uintptr_t ioq_slot;
 #define IOQ_SKIP_ONE (~IOQ_BLOCKED)
 
 // Need room for two flag bits
-bfs_static_assert(alignof(struct ioq_ent) > 2);
+bfs_static_assert(alignof(struct ioq_ent) >= (1 << 2));
 
 /**
  * An MPMC queue of I/O commands.

src/ioq.h

@@ -32,12 +32,19 @@ enum ioq_op {
 	IOQ_STAT,
 };
 
+/**
+ * The I/O queue implementation needs two tag bits in each pointer to a struct
+ * ioq_ent, so we need to ensure at least 4-byte alignment.  The natural
+ * alignment is enough on most architectures, but not m68k, so over-align it.
+ */
+#define IOQ_ENT_ALIGN alignas(4)
+
 /**
  * An I/O queue entry.
  */
 struct ioq_ent {
 	/** The I/O operation. */
-	enum ioq_op op;
+	IOQ_ENT_ALIGN enum ioq_op op;
 
 	/** The return value (on success) or negative error code (on failure). */
 	int result;