Removing stray xori that came from big endian copy/paste.
Adding atomicand8 check to runtime.check() that would have revealed
this error.
Might fix #19396.

Change-Id: If8d6f25d3e205496163541eb112548aa66df9c2a
Reviewed-on: https://go-review.googlesource.com/38257
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>