Fix alignment of stack variable

Declare with union similar to PGAlignedBlock.

Report and fix by Andres Freund

Discussion: https://postgr.es/m/20240407190731.izm3mdazednrsiqk%40awork3.anarazel.de

src/backend/access/common/tidstore.c

@@ -302,9 +302,13 @@ TidStoreDestroy(TidStore *ts)
 void
 TidStoreSetBlockOffsets(TidStore *ts, BlockNumber blkno, OffsetNumber *offsets,
 						int num_offsets)
+{
+	union
 	{
 		char		data[MaxBlocktableEntrySize];
-	BlocktableEntry *page = (BlocktableEntry *) data;
+		BlocktableEntry force_align_entry;
+	}			data;
+	BlocktableEntry *page = (BlocktableEntry *) data.data;
 	bitmapword	word;
 	int			wordnum;
 	int			next_word_threshold;