Change len in syncicache(_, len) from int to size_t

Change len in syncicache(_, len) from int to size_t

The powerpc64 part is under #if 0, so this change affects only macppc.
Simplify powerpc64's __syncicache (which had size_t len) and copy it
to macppc's syncicache (which had int len).

macppc was looping while ((l -= CACHELINESIZE) > 0).  The loop would
be infinite if l became an unsigned type like size_t.  It is simpler
to set size_t i = 0, do i += by, and loop while (i < len).  It helps
that dcbst and icbi can add 2 registers, from + i.