Do not lock in BufferGetLSNAtomic() on archs with 8 byte atomic reads

Do not lock in BufferGetLSNAtomic() on archs with 8 byte atomic reads

On platforms where we can read or write the whole LSN atomically, we do
not need to lock the buffer header to prevent torn LSNs. We can do this
only on platforms with PG_HAVE_8BYTE_SINGLE_COPY_ATOMICITY, and when the
pd_lsn field is properly aligned.

For historical reasons the PageXLogRecPtr was defined as a struct with
two uint32 fields. This replaces it with a single uint64 value, to make
the intent clearer. To prevent issues with weak typedefs the value is
still wrapped in a struct.

This also adjusts heapfuncs() in pageinspect, to ensure proper alignment
when reading the LSN from a page on alignment-sensitive hardware.

Idea by Andres Freund. Initial patch by Andreas Karlsson, improved by
Peter Geoghegan. Minor tweaks by me.

Author: Andreas Karlsson <andreas@proxel.se>
Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Tomas Vondra <tomas@vondra.me>
Discussion: https://postgr.es/m/b6610c3b-3f59-465a-bdbb-8e9259f0abc4@proxel.se