Fix MemoryContextAllocAligned's interaction with Valgrind.

Fix MemoryContextAllocAligned's interaction with Valgrind.

Arrange that only the "aligned chunk" part of the allocated space is
included in a Valgrind vchunk.  This suppresses complaints about that
vchunk being possibly lost because PG is retaining only pointers to
the aligned chunk.  Also make sure that trailing wasted space is
marked NOACCESS.

As a tiny performance improvement, arrange that MCXT_ALLOC_ZERO zeroes
only the returned "aligned chunk", not the wasted padding space.

In passing, fix GetLocalBufferStorage to use MemoryContextAllocAligned
instead of rolling its own implementation, which was equally broken
according to Valgrind.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/285483.1746756246@sss.pgh.pa.us