C11 alignas instead of unions -- extended alignments

This replaces some uses of pg_attribute_aligned() with the standard
alignas() for cases where extended alignment (larger than max_align_t)
is required.

This patch stipulates that all supported compilers must support
alignments up to PG_IO_ALIGN_SIZE, but that seems pretty likely.

We can then also desupport the case where direct I/O is disabled
because pg_attribute_aligned is not supported.

Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/46f05236-d4d4-4b4e-84d4-faa500f14691%40eisentraut.org

src/include/c.h

@@ -1132,25 +1132,15 @@ typedef struct PGAlignedBlock
  * for I/O in general, but may be strictly required on some platforms when
  * using direct I/O.
  */
-typedef union PGIOAlignedBlock
+typedef struct PGIOAlignedBlock
 {
-#ifdef pg_attribute_aligned
-	pg_attribute_aligned(PG_IO_ALIGN_SIZE)
-#endif
-	char		data[BLCKSZ];
-	double		force_align_d;
-	int64		force_align_i64;
+	alignas(PG_IO_ALIGN_SIZE) char data[BLCKSZ];
 } PGIOAlignedBlock;
 
 /* Same, but for an XLOG_BLCKSZ-sized buffer */
-typedef union PGAlignedXLogBlock
+typedef struct PGAlignedXLogBlock
 {
-#ifdef pg_attribute_aligned
-	pg_attribute_aligned(PG_IO_ALIGN_SIZE)
-#endif
-	char		data[XLOG_BLCKSZ];
-	double		force_align_d;
-	int64		force_align_i64;
+	alignas(PG_IO_ALIGN_SIZE) char data[XLOG_BLCKSZ];
 } PGAlignedXLogBlock;
 
 /* msb for char */

src/include/storage/fd.h

@@ -85,10 +85,9 @@ extern PGDLLIMPORT int max_safe_fds;
  * to the appropriate Windows flag in src/port/open.c.  We simulate it with
  * fcntl(F_NOCACHE) on macOS inside fd.c's open() wrapper.  We use the name
  * PG_O_DIRECT rather than defining O_DIRECT in that case (probably not a good
- * idea on a Unix).  We can only use it if the compiler will correctly align
- * PGIOAlignedBlock for us, though.
+ * idea on a Unix).
  */
-#if defined(O_DIRECT) && defined(pg_attribute_aligned)
+#if defined(O_DIRECT)
 #define		PG_O_DIRECT O_DIRECT
 #elif defined(F_NOCACHE)
 #define		PG_O_DIRECT 0x80000000