On machines that allow for unaligned access,  speed up hash ops
with known key length at the expense of those where the key length
is unknown.

Background: Efficient hash functions computation is essential for
performance in many parts of SVN.  Many of the frequently callers
them have already been changed to provide the key length in.

* subversion/libsvn_subr/hash.c
  (hashfunc_compatible): use a slightly different but faster
                         formula when the CPU supports that


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1483434 13f79535-47bb-0310-9956-ffa450edef68