Merge r1470348:

Fix amd64 assembler version of apr_atomic_xchgptr()

PR: 51851
Submitted by: Mattias Engdegård <mattiase acm org>


git-svn-id: https://svn.apache.org/repos/asf/apr/apr/branches/1.4.x@1470350 13f79535-47bb-0310-9956-ffa450edef68