Use C11 alignas in pg_atomic_uint64 definitions

They were already using pg_attribute_aligned.  This replaces that with
alignas and moves that into the required syntactic position.  This
ends up making these three atomics implementations appear a bit more
consistent, but shouldn't change anything otherwise.

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/port/atomics/arch-ppc.h

@@ -36,7 +36,7 @@ typedef struct pg_atomic_uint32
 #define PG_HAVE_ATOMIC_U64_SUPPORT
 typedef struct pg_atomic_uint64
 {
-	volatile uint64 value pg_attribute_aligned(8);
+	alignas(8) volatile uint64 value;
 } pg_atomic_uint64;
 
 #endif

src/include/port/atomics/generic-gcc.h

@@ -100,10 +100,9 @@ typedef struct pg_atomic_uint32
 	&& (defined(HAVE_GCC__ATOMIC_INT64_CAS) || defined(HAVE_GCC__SYNC_INT64_CAS))
 
 #define PG_HAVE_ATOMIC_U64_SUPPORT
-
 typedef struct pg_atomic_uint64
 {
-	volatile uint64 value pg_attribute_aligned(8);
+	alignas(8) volatile uint64 value;
 } pg_atomic_uint64;
 
 #endif /* defined(HAVE_GCC__ATOMIC_INT64_CAS) || defined(HAVE_GCC__SYNC_INT64_CAS) */

src/include/port/atomics/generic-msvc.h

@@ -37,9 +37,9 @@ typedef struct pg_atomic_uint32
 } pg_atomic_uint32;
 
 #define PG_HAVE_ATOMIC_U64_SUPPORT
-typedef struct pg_attribute_aligned(8) pg_atomic_uint64
+typedef struct pg_atomic_uint64
 {
-	volatile uint64 value;
+	alignas(8) volatile uint64 value;
 } pg_atomic_uint64;