Optimize tile (mostly tilegx) memcpy and memmove performance.

Optimize tile (mostly tilegx) memcpy and memmove performance.

- Override <memcopy.h> so we use full 8-byte word copies on tilegx32
  for memmove, then use op_t in memcpy instead of the previous
  locally-defined word_t just to avoid proliferating identical types.
- Fix bug in memcpy prefetch that caused us to never prefetch past
  the first cache line.
- Optimize misaligned memcpy by inlining _wordcopy_fwd_dest_aligned
  instead of just doing a dumb word-at-a-time copy.
- Make memcpy safe for forward copies by doing all the loads from
  a given cache line prior to doing a wh64 (cache line zero-fill)
  on the destination.  Remove now-redundant src == dst check.
- Copy and optimize the generic wordcopy.c routines to use the tile
  "double align" instruction instead of the MERGE macro; to avoid
  offset addressing mode (which tile doesn't have) by rewriting the
  pointer math to load and store with a zero index; and to use
  post-increment addresses in the inner loops to improve scheduling.