powerpc64le: add optimized strlen for P9

powerpc64le: add optimized strlen for P9

This started as a trivial change to Anton's rawmemchr.  I got
carried away.  This is a hybrid between P8's asympotically
faster 64B checks with extremely efficient small string checks
e.g <64B (and sometimes a little bit more depending on alignment).

The second trick is to align to 64B by running a 48B checking loop
16B at a time until we naturally align to 64B (i.e checking 48/96/144
bytes/iteration based on the alignment after the first 5 comparisons).
This allieviates the need to check page boundaries.

Finally, explicly use the P7 strlen with the runtime loader when building
P9.  We need to be cautious about vector/vsx extensions here on P9 only
builds.