pgsql: Declare aarch64 has single copy atomicity for 8 byte values.

Declare aarch64 has single copy atomicity for 8 byte values.

Architecture Reference Manual for ARMv8 B2.2.1 [1]

For explicit memory effects generated from an Exception level the
following rules apply:
- A read that is generated by a load instruction that loads a single
general-purpose register and is aligned to the size of the read in the
instruction is single-copy atomic.
- A write that is generated by a store instruction that stores a single
general-purpose register and is aligned to the size of the write in the
instruction is single-copy atomic.

[1] https://documentation-service.arm.com/static/61fbe8f4fa8173727a1b734e
https://developer.arm.com/documentation/ddi0487/latest

Author: Yura Sokolov <y(dot)sokolov(at)postgrespro(dot)ru>
Reviewed-by: Thomas Munro <thomas(dot)munro(at)gmail(dot)com>
Reviewed-by: Alexander Korotkov <aekorotkov(at)gmail(dot)com>
Discussion: https://postgr.es/m/f7f3f0febe27862711f924a7b0f39e065e547f4b.camel%40postgrespro.ru
Discussion: https://postgr.es/m/CA%2BhUKGKyJf7kwYkqDgzTE26Ra1m9nvM%3Deds2RSSu7WSL-r2wKw%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b2397aae23982b77de29d3a55b263a03852d0714

Modified Files
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src/include/port/atomics/arch-arm.h | 6 ++++++
1 file changed, 6 insertions(+)