runtime/internal/atomic: improve ARM atomics

This is a follow-up of CL 93637. There, when we redirect sync/atomic
to runtime/internal/atomic, a few good implementations of ARM atomics
were lost. This CL brings most of them back, with some improvements.

- Change atomic Store to a plain store with memory barrier, as we
  already changed atomic Load to plain load with memory barrier.

- Use native 64-bit atomics on ARMv7, jump to Go implementations
  on older machines. But drop the kernel helper. In particular,
  for Load64, just do loads, not using Cas on the address being
  load from, so it works also for read-only memory (since we have
  already fixed 32-bit Load).

Change-Id: I725cd65cf945ae5200db81a35be3f251c9f7af14
Reviewed-on: https://go-review.googlesource.com/111315


Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>