pgsql: Add pg_atomic_unlocked_write_u64

Add pg_atomic_unlocked_write_u64

The 64bit equivalent of pg_atomic_unlocked_write_u32(), to be used in an
upcoming patch converting BufferDesc.state into a 64bit atomic.

Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Discussion: https://postgr.es/m/fvfmkr5kk4nyex56ejgxj3uzi63isfxovp2biecb4bspbjrze7@az2pljabhnff

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/7902a47c20b1d5c0b7d20071f9ada6a0185bf39b

Modified Files
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src/include/port/atomics.h         | 10 ++++++++++
src/include/port/atomics/generic.h |  9 +++++++++
2 files changed, 19 insertions(+)