Merge the r1483575 group from trunk:

 * r1483575, r1483580, r1485018
   Define SVN_UNALIGNED_ACCESS_IS_OK for PowerPC.
   Justification:
     Performance for free.
   Votes:
     +1: mattiase (non-binding, not including r1485018)
     +1: breser, stefan2, rhuijben


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.8.x@1489282 13f79535-47bb-0310-9956-ffa450edef68