Use PGAlignedXLogBlock for some code simplification

Use PGAlignedXLogBlock for some code simplification

The code in BootStrapXLOG() and in pg_test_fsync.c tried to align WAL
buffers in complicated ways.  Also, they still used XLOG_BLCKSZ for
the alignment, even though that should now be PG_IO_ALIGN_SIZE.  This
can now be simplified and made more consistent by using
PGAlignedXLogBlock, either directly in BootStrapXLOG() and using
alignas in pg_test_fsync.c.

Reviewed-by: Chao Li <[email protected]>
Reviewed-by: Thomas Munro <[email protected]>
Discussion: https://www.postgresql.org/message-id/flat/f462a175-b608-44a1-b428-bdf351e914f4%40eisentraut.org