pgsql: 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 <li(dot)evan(dot)chao(at)gmail(dot)com>
Reviewed-by: Thomas Munro <thomas(dot)munro(at)gmail(dot)com>
Discussion: https://www.postgresql.org/message-id/flat/f462a175-b608-44a1-b428-bdf351e914f4%40eisentraut.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/804046b39a27973751c920f8f41504697f3f52d5

Modified Files
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src/backend/access/transam/xlog.c | 12 ++++--------
src/bin/pg_test_fsync/pg_test_fsync.c | 11 ++++-------
2 files changed, 8 insertions(+), 15 deletions(-)