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/memchr_64.c

@@ -36,7 +36,7 @@ void *memchr(const void *s, int c, size_t n)
 	p = (const uint64_t *)(s_int & -8);
 
 	/* Create eight copies of the byte for which we are looking. */
-	goal = 0x0101010101010101ULL * (uint8_t) c;
+	goal = copy_byte(c);
 
 	/* Read the first word, but munge it so that bytes before the array
 	 * will not match goal.