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/strchr_64.c

@@ -26,7 +26,7 @@ char *strchr(const char *s, int c)
 	const uint64_t *p = (const uint64_t *)(s_int & -8);
 
 	/* Create eight copies of the byte for which we are looking. */
-	const uint64_t goal = 0x0101010101010101ULL * (uint8_t) c;
+	const uint64_t goal = copy_byte(c);
 
 	/* Read the first aligned word, but force bytes before the string to
 	 * match neither zero nor goal (we make sure the high bit of each