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

Author: macdice

src/include/port/atomics/arch-arm.h

@@ -23,4 +23,10 @@
  */
 #if !defined(__aarch64__) && !defined(__aarch64)
 #define PG_DISABLE_64_BIT_ATOMICS
+#else
+/*
+ * Architecture Reference Manual for ARMv8 states aligned read/write to/from
+ * general purpose register is atomic.
+ */
+#define PG_HAVE_8BYTE_SINGLE_COPY_ATOMICITY
 #endif  /* __aarch64__ || __aarch64 */