sync/atomic, runtime/internal/atomic: don't assume reads from 0 fail

For a misaligned address force a panic rather than assuming that reading
from the address 0 will cause one.

Change-Id: I25f32be86120d6a4fd33db01f159c5fadac9674e
Reviewed-on: https://go-review.googlesource.com/69850
Reviewed-by: Ian Lance Taylor <iant@golang.org>