Relax shareability domain for dmb st in AO_nop_write (ARM/AArch64)

* src/atomic_ops/sysdeps/gcc/aarch64.h (AO_nop_write): Replace "dmb st"
to "dmb ishst" (this way the barriers will occupy the inner shareable
domain).
* src/atomic_ops/sysdeps/gcc/arm.h (AO_nop_write): Likewise.
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