Add versions of wcscpy, wcschr, wcsrchr for power6/power7.

Add versions of wcscpy, wcschr, wcsrchr for power6/power7.

Initially based on the versions found in wcsmbs/* ; these files have
been changed by hand unrolling, and adding some additional variables
to allow some read-ahead to occur, which then relieves some of the
wait-for-increment/wait-for-load/wait-for-compare-results pressure
that was slowing down every iteration through the while-loop.

For 64-bit Power7, These changes give an approx 20% throughput boost
for the wcschr and wcsrchr functions; and approx 40% boost for the
wcscpy function.  32-bit improvements appear to be slightly better
with ~ %30 and ~ %45 respectively.  Results for Power6 closely match
those for power7.