tile: optimize and clean up string functions

This change cleans up the string code in a number of ways:

- For memcpy(), fix bug in prefetch and increase distance to 3 lines;
  optimize for unaligned data; do all loads before wh64 to make memcpy
  safe for forward-overlapping calls; etc.  Performance is improved.

- Use new copy_byte() function on tilegx to spread a single byte value
  out into a full word using the shufflebytes instruction.

- Clean up header include ordering to be more canonical, and remove
  spurious #undefs of function names.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>

arch/tile/lib/memset_32.c

@@ -12,13 +12,10 @@
  *   more details.
  */
 
-#include <arch/chip.h>
-
 #include <linux/types.h>
 #include <linux/string.h>
 #include <linux/module.h>
-
-#undef memset
+#include <arch/chip.h>
 
 void *memset(void *s, int c, size_t n)
 {