[COMMITTERS] pgsql: Fix shm_toc.c to always return buffer-aligned memory.

Fix shm_toc.c to always return buffer-aligned memory.

Previously, if you passed a non-aligned size to shm_toc_create(), the
memory returned by shm_toc_allocate() would be similarly non-aligned.
This was exposed by commit 3cda10f41b, which allocated structs containing
a pg_atomic_uint64 field with shm_toc_allocate(). On systems with
MAXIMUM_ALIGNOF = 4, such structs still need to be 8-bytes aligned, but
the memory returned by shm_toc_allocate() was only 4-bytes aligned.

It's quite bogus that we abuse BUFFERALIGN to align the structs for
pg_atomic_uint64. It doesn't really have anything to do with buffers. But
that's a separate issue.

This ought to fix the buildfarm failures on 32-bit x86 systems.

Discussion: https://www.postgresql.org/message-id/7e0a73a5-0df9-1859-b8ae-9acf122dc38d@iki.fi

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ac883ac453e9c479f397780918f235c440b7a02f

Modified Files
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src/backend/storage/ipc/shm_toc.c | 24 +++++++++++++++++++-----
src/include/c.h                   |  1 +
2 files changed, 20 insertions(+), 5 deletions(-)