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atomic.Storep1 is not supposed to invoke a write barrier (that's what
atomicstorep is for), but currently does on s390x. This causes a panic
in runtime.mapzero when it tries to use atomic.Storep1 to store what's
actually a scalar.

Fix this by eliminating the write barrier from atomic.Storep1 on
s390x. Also add some documentation to atomicstorep to explain the
difference between these.

Fixes #15270.

Change-Id: I291846732d82f090a218df3ef6351180aff54e81
Reviewed-on: https://go-review.googlesource.com/21993

Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Michael Munday <munday@ca.ibm.com>